Relict Non-Glacial Surfaces and Autochthonous Blockfields in the Northern Swedish Mountains
Bradley W. Goodfellow
Doctoral Dissertation 2008 Department of Physical Geography and Quaternary Geology Stockholm University © Bradley W. Goodfellow ISSN: 1653-7211 ISBN: 978-91-7155-620-2
Paper I © Elsevier Paper II © Elsevier
Layout: Bradley W. Goodfellow (except for Papers I and II)
Cover photo: View of relict non-glacial surfaces, looking SE from the blockfield-mantled Tarfalatjårro summit, northern Swedish mountains (Bradley Goodfellow, August 2006)
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Abstract
Relict non-glacial surfaces occur in many formerly glaciated landscapes, where they represent areas that have escaped significant glacial modification. Frequently distinguished by blockfield mantles, relictnon- glacial surfaces are important archives of long-term weathering and landscape evolution processes. The aim of this thesis is to examine the distribution, weathering, ages, and formation of relict non-glacial surfaces in the northern Swedish mountains. Mapping of surfaces from aerial photographs and analysis in a GIS revealed five types of relict non-glacial surfaces that reflect differences in surface process types or rates according to elevation, gradient, and bedrock lithology. Clast characteristics and fine matrix granulometry, chemistry, and mineralogy reveal minimal chemical weathering of the blockfields. Terrestrial cosmogenic nuclides were measured in quartz samples from two blockfield-mantled summits and a numerical ice sheet model was applied to account for periods of surface burial beneath ice sheets and nuclide production rate changes attributable to glacial isostasy. Total surface histories for each summit are almost certainly, but not unequivocally, confined to the Quaternary. Maximum modelled erosion rates are as low as 4.0 mm kyr-1, which is likely to be near the low extreme for relict non-glacial surfaces in this landscape. The blockfields of the northern Swedish mountains are Quaternary features formed through subsurface physical weathering processes. While there is no need to appeal to Neogene chemical weathering to explain blockfield origins, these surfaces have remained continuously regolith-mantled and non-glacial since their inception. Polygenetic surface histories are therefore indicated, where the large-scale surface morphologies are potentially older than their regolith mantles.
Keywords: blockfield, chemical weathering, cosmogenic exposure dating, glaciated landscape, northern Sweden, periglacial geomorphology, relict surface 2 B.W. Goodfellow / Doctoral Dissertation / 2008
This doctoral thesis consists of a summary and four papers:
Paper I
Goodfellow, B.W., 2007. Relict non-glacial surfaces in formerly glaciated landscapes. Earth-Science Reviews 80, 47-73.
Reprinted with permission of Elsevier.
Paper II
Goodfellow, B.W., Stroeven, A.P., Hättestrand, C., Kleman, J., Jansson, K.N., 2008. Deciphering a non- glacial/glacial landscape mosaic in the northern Swedish mountains. Geomorphology 93, 213-232.
Reprinted with permission of Elsevier.
Paper III
Goodfellow, B.W., Fredin, O., Derron, M.-H., Stroeven, A.P. Weathering processes and Quaternary origin of an alpine blockfield in Arctic Sweden. Manuscript.
Paper IV
Goodfellow, B.W., Stroeven, A.P., Fabel, D., Fredin, O., Derron, M.-H., Bintanja, R., Caffee, M.W., Freeman, S.P.H.T. Cosmogenic radionuclide and fine matrix analyses of blockfields, northern Sweden: Indications of Quaternary origins. Manuscript.
I initiated this project with Arjen Stroeven and project planning benefited from additional inputs by Clas Hättestrand, Johan Kleman, and Ola Fredin. I completed aerial photograph interpretation, subsequent mapping, and GIS analysis of relict non-glacial surfaces in Paper II. I completed fieldwork for Papers II – IV with assistance from Arjen, Ola, and field assistants acknowledged in each article. I analysed grain size and X-ray diffraction data with Marc-Henri Derron for Papers III and IV, and X-ray fluorescence data for Paper III, also with Marc-Henri. Scanning electron microscopy analyses for paper III were performed by Ola and Marc- Henri, and for paper IV by Marc-Henri and I. Derek Fabel, Maria Miguens-Rodriguez ,and I prepared quartz samples for accelerator mass spectrometry in Paper IV (Australian National University and Scottish Universities Environmental Research Centre), with subsequent terrestrial cosmogenic radionuclide interpretation performed by Derek and I. Clas, Johan, Krister Jansson, Ola, and Marc-Henri were all involved in fieldwork planning and discussion of results. I wrote all articles, with detailed reviews provided by Arjen. Additional reviews for Paper I were provided by Clas and Johan; for Paper II by Clas, Johan, and Krister; for Paper III by Ola; and for Paper IV by Derek, Ola, Marc-Henri, and Marc Caffee. Relict surfaces and blockfields / northern Swedish mountains 3
Introduction 2005; Fjellanger et al., 2006; Linge et al., 2006; Phillips et al., 2006; Goering et al., 2008; Jansson Relict surfaces in present landscapes were initiated et al., in review). The spatial distribution of relict under different environmental and geomorphological non-glacial surfaces may also reveal information conditions than those prevailing in the landscapes on glacial period climates (Munroe, 2006). today (Battiau-Queney, 1996; Migoń and Goudie, 2. Determining how and at what rates surfaces evolve 2001). Whereas most surfaces in formerly glaciated over long time spans (Small et al., 1999; Anderson, landscapes bear strong imprints of glacial processes, 2002; Staiger et al., 2005; Babault et al., 2007). some may show little or no sign of glacial modification. 3. Potentially determining interstadial, interglacial, These surfaces, here referred to as relict non-glacial and/or preglacial environmental conditions from, surfaces, have preglacial origins and have developed for example, fine matrix materials contained further under ice-free conditions during interglacials, within relict non-glacial surface regolith (Hall, interstadials, or on nunataks (Kleman and Borgström, 1985; Rea et al., 1996a). Formerly glaciated 1990; Rea et al., 1996b; Ballantyne et al., 1997). landscapes currently occupy a range of climatic Relict non-glacial surfaces are most readily identified zones including polar (Stroeven and Kleman, as periglaciated, low relief, upland surfaces separated 1999; Sugden et al., 2005), tundra (Fjellanger et from lower, more recent, glacial surfaces by sharply al., 2006), maritime (Whalley et al., 1981; Paasche defined boundaries, such as trough walls (Brunsden, et al., 2006), continental (Small et al., 1999; 1993; Ollier and Pain, 1997; Stroeven and Kleman, Stroeven et al., 2002a; Munroe, 2006), alpine 1999; Migoń and Goudie, 2001; Anderson, 2002; (Dahl, 1966; Rea et al., 1996b), and cool temperate Munroe, 2006). Relict non-glacial surfaces have (Hall, 1986; Olvmo et al., 2005). Most continue been identified in formerly glaciated landscapes of to be subject to a periglacial regime (Kleman and Scandinavia (Ahlmann, 1919; Gjessing, 1967; Nesje Stroeven, 1997; Dredge, 2000; Marquette et al., et al., 1988; Kleman and Borgström, 1990; Rea et 2004; Fjellanger et al., 2006; Munroe, 2006). al., 1996b; Kleman and Stroeven, 1997; Lidmar- Knowledge of preglacial climates is limited with Bergström et al., 2000; Hättestrand and Stroeven, a number of studies speculating on warm humid 2002; Fjellanger et al., 2006; Paasche et al., 2006), conditions (Roaldset et al., 1982; Hall, 1986; Rea Scotland (Hall, 1985; Ballantyne et al., 1997; Phillips et al., 1996b; Marquette et al., 2004; Paasche et al., et al., 2006), Svalbard (Jonsson, 1983; Landvik et al., 2006). Relict non-glacial surfaces may maintain 2003), Greenland (Sugden, 1974), North America climatic evidence from different periods due (Ives, 1958; Sugden and Watts, 1977; Bierman et to regional differences in denudation histories. al., 1999; Briner et al., 2003; Marquette et al., 2004; Davis et al., 2006), and Antarctica (Sugden et al., Autochthonous blockfields, which are coarse, 2005). This thesis examines relict non-glaciated blocky mantles produced from in situ weathering, surfaces in the formerly glaciated northern Swedish are key features of many relict non-glacial surfaces. mountains. However, the origins of blockfields remain elusive Relict non-glacial surfaces are important for: and they are usually considered, in periglaciated landscapes, to represent remnants of pre-Quaternary 1. Inferring aspects of regional glacial history, such deep weathering profiles (Caine, 1968; Ives, 1974; as thicknesses of former ice sheets, basal thermal Clapperton, 1975; Nesje et al., 1988; Rea et al., 1996b; regimes, glacial erosion patterns, and relief Boelhouwers et al., 2002; André, 2003; Marquette et production (Hall and Sugden, 1987; Nesje et al., al., 2004; Fjellanger et al., 2006; Paasche et al., 2006; 1988; Sugden, 1989; Kleman, 1994; Nesje and André et al., 2008). According to this model, block Whillans, 1994; Ballantyne et al., 1997; Glasser production was initiated through chemical weathering and Hall, 1997; Kleman and Stroeven, 1997; Small under a comparatively warm Neogene climate. and Anderson, 1998; Bierman et al., 1999; Clarhäll Subsequent soil stripping occurred during the colder and Kleman, 1999; Kleman et al., 1999; Stroeven Quaternary and the chemically weakened bedrock was et al., 2002b; Briner et al., 2003, 2005; Marquette frost shattered and periglacially reworked to produce et al., 2004; Staiger et al., 2005; Sugden et al., blockfield mantles. The presence of saprolite and/ 4 B.W. Goodfellow / Doctoral Dissertation / 2008 or substantial quantities of secondary minerals, and processes of landscape evolution, few studies an assumed dependence of chemical weathering on have directly targeted the long-term, non-glacial a warm climate, are critical to this interpretation of development of these surfaces. Small et al. (1999) blockfield origins. and Staiger et al. (2005) used terrestrial cosmogenic Only a few recent studies have invoked a nuclides (TCNs) to measure surface erosion Quaternary origin for blockfields (Kleman and rates, Anderson (2002) and Babault et al. (2007) Borgström, 1990; Ballantyne, 1998; Ballantyne et al., characterised their morphological development 1998), where blocks are produced through physical through numerical modelling, and Bonow et al. weathering processes, such as frost shattering, and, (2003, 2007) analysed relict non-glacial surface again, periglacially reworked to produce coarse morphological development according to slope angles. mantles. Quantities of secondary minerals are low Consequently, the processes acting on these surfaces, and saprolite is absent. Although, at least some, particularly those that are blockfield-mantled, the blockfields may result from Quaternary periglacial long-term rates at which these processes operate, and weathering, a critical issue is that, with the exception the resulting large-scale morphological evolution of of blockfields forming in highly frost susceptible these surfaces, remain only partly known. limestone in the Canadian Arctic (Dredge, 1992), The aim of this thesis is to examine the distribution, there appear to be none that are presently forming characteristics, weathering, and ages of relict non- in periglaciated landscapes (Boelhouwers, 2004). glacial surfaces in the northern Swedish mountains Blockfields are therefore largely relict features that to better understand their significance to long-term seem to depend on a climate or weathering process landscape development in formerly glaciated regions. that is different to those presently occurring in The study will be conducted over a range of spatial periglaciated landscapes. scales (Fig. 1), beginning with a global perspective through a literature review, then progressing Study Rational and Objectives to a regional scale through remote sensing and geographical information system (GIS) analyses of Most previous work on relict non-glacial surfaces, the northern Swedish mountains. However, the main particularly in the northern Swedish mountains, has focus of this study will be at the regolith scale, where focussed on the information they can provide on blockfield weathering and ages will be examined, former ice sheet dimensions and basal thermal regimes with intra-site and inter-site variations in weathering or on using them as references for the development also being explored. The regolith zone has been of glacial landforms and relief (Sugden, 1974, 1978; labelled the critical zone (NRC, 2001), partly because Hall and Sugden, 1987; Nesje et al., 1988; Kleman most surfaces, including relict non-glacial surfaces, and Borgström, 1990; Kleman, 1994; Nesje and are regolith-mantled, resulting in small-scale regolith Whillans, 1994; Ballantyne et al., 1997; Glasser and weathering and erosional processes being fundamental Hall, 1997; Kleman and Stroeven, 1997; Small and to large-scale landscape evolution. Studying relict Anderson, 1998; Clarhäll and Kleman, 1999; Kleman non-glacial surfaces at the regolith scale is therefore et al., 1999; Hättestrand and Stroeven, 2002; André, an appropriate starting point to understanding their 2004; Staiger et al., 2005; Phillips et al., 2006; Jansson significance to landscape development in formerly et al., in review). Cosmogenic dating has confirmed glaciated regions. that relict non-glacial surfaces pre-date the last glaciation (Bierman et al., 1999; Briner et al., 2003; Study Location Marquette et al., 2004; Fjellanger et al., 2006; Linge et al., 2006; Linge et al., 2007), with preservation The study area is situated between 66°46’ and in northern Sweden widely attributable to coverage 68°37’N in the northern Swedish mountains (Fig. 2). by non-erosive, cold-based ice sheets (Kleman et Encompassed within this area are the highest parts of al., 1997, 1999; Fabel et al., 2002; Hättestrand and the northern Scandes, including the Kebnekaise (2113 Stroeven, 2002; Stroeven et al., 2002a). m a.s.l.) and Sarek (2080 m a.s.l.) massifs. Central However, despite these surfaces being potentially and western parts of the study area are composed of important archives of long-term weathering and Caledonian nappes, with the eastern part formed from Relict surfaces and blockfields / northern Swedish mountains 5
Paper III Paper I Regolith scale - Global scale - literature review Intra-site variations in blockfield weathering, intensive sampling strategy in two pits (Tarfalatjårro summit and saddle)
Paper IV Regolith scale - Inter-site variations in blockfield weathering Paper II 3 hillslope transects Regional scale - - Tarfalatjårro Remote sensing/GIS - Alddasčorru analysis of the northern Swedish mountains - Duoptečohkka
Fig. 1. A spatial representation of the treatment of relict non-glacial surfaces in the four papers comprising this thesis.
Precambrian intrusive and volcanic rocks (Kulling, Methods 1964). The Arctic maritime climate of Norway and the continental climate of northern Sweden converge Landscape analysis was performed over three within the study area. Discontinuous permafrost spatial scales. Mapping and analysis of relict non- persists on high surfaces (King, 1986; Isaksen et glacial surfaces (Paper II) was completed for the al., 2001) and the current annual maximum active entire 24 082 km2 area contained in Figure 2. Using a layer depth in the northern saddle of Tarfalatjårro stereoscope, relict non-glacial surfaces were classified is 1.5–1.6 m (Isaksen et al., 2007). Although from 1:60 000 colour infrared aerial photographs, current glaciation is confined to small cirque and according to (i) large-scale morphologies, including valley glaciers in the higher massifs, the study area rounded summits and fluvial valleys, (ii) signs of has been repeatedly inundated by the Quaternary extensive subaerial weathering, including blockfield ice sheets, with cirque glaciation inferred to have mantles, tors, and roughened exposed bedrock, and been dominant before 2.0 million years ago (Myr), (iii) surface expressions of dominant periglacial mountain ice sheets dominant between 0.7 and 2.0 processes, such as soil creep, solifluction, and Myr, and Fennoscandian ice sheets developing over nivation. The relict non-glacial surface classes were the last 0.7 Myr (Kleman and Stroeven, 1997). This then imposed on a 50 m digital elevation model area was selected for study because the repeated (DEM) and analysed in a GIS for patterns in their formation of cold-based ice during the Quaternary distribution according to bedrock lithology, elevation, glaciations preserved large areas of an essentially and slope gradient. Field control was provided by non-glacial landscape, with only superficial glacial helicopter surveys of the northern part of the study modification (Kleman and Stroeven, 1997; Clarhäll area from Nulpotjåkka to Alddasčorru, and ground and Kleman, 1999; Kleman et al., 1999; Fabel et al., investigations, including hand digging of regolith 2002; Stroeven et al., 2002b). pits, at Alddasčorru, Duoptečohkka, Ruohtahakčorru, Tarfalatjårro, Tjeurolako, Kebnetjåkka, and Nulpotjåkka (Fig. 2). Additional ground surveys, without pit digging, were completed at Luovárri, Geargevuomoš, and Njulla. 6 B.W. Goodfellow / Doctoral Dissertation / 2008 Contour Contour interval: 20 m Till XRD Cosmogenics Cosmogenics and XRD XRD Sections Legend Legend
1 1500 1500
Pit excavations: excavations: Pit
1100 1100
24’ 24’ 19 o 2 1 km Tarfalatjårro transect Alddasčorru transect 3 7 6
5 1200 1200
Alddasčorru transect
4
1300 1300 1300 1300 500 m
1400 0
22’ 22’ 19
1500 1500 o 1 2
2 1500 1500 1600 1600 3 0 3 1
1200
39’ 39’ 18 o 4 4 Duoptečohkka transect 7 6 5 7 5 6
1100 Tarfalatjårro transect (1626 m) 9 8 Tarfalajårro Tarfalajårro
+ 1600 1600 25’ 25’ 55’ 55’ 8 o o 9 Duoptečohkka transect 10 68 67 permafrost monitoring borehole (Isaksen et al., 2001, 2007) Km N N 68 68 40
(1538 m) (1538 m) N N (~ 850 m) (~ 850 m) Alddasčorru Alddasčorru ) m Geargevuomoš Geargevuomoš
0 30 Luovárri (1230 m) (1230 m) (~1336 m) (~1336 m) 6 (5 Duoptečohkka Duoptečohkka re u ja s
a Amphibolite/dolerite Calcareous/mica schist Gneiss, mica schist Intrusive rocks Metasediments (Misc.) Sedimentary rocks Volcanic rocks Hydrology t
u 20
E E E 19 19 a 30’N 19
R Bedrock Geology 67 67
) m 0 (1349 m) (1349 m) 50
gi (~ 10 Ultevis Ultevis E E E 19 19 g Ruohtahakčorru Ruohtahakčorru á (~1000 m) sv (~1000 m) a tt is (1178 m) (1178 m) V (~1200 m) (~1200 m) Tjeurolako Tjeurolako 0 (341 m) (341 m) Viddjáčohkka Viddjáčohkka Torneträsk Tarfala Station Station (1140 m) (1140 m) Research Research Abisko (380 m) (380 m) Njulla (1626 m) (1626 m) (1164 m) Fig. 6 Tarfalatjårro Tarfalatjårro Palkastak (1550 m) (1550 m) Kebnetjåkka (~1440 m) (~1440 m)
(981 m) (981 m) )
Njirávtjåhkkå Njirávtjåhkkå (2113 m) (2113 m) E E E E 18 m 18 Latnjavággi Latnjavággi (1405 m) (1405 m) Kebnekaise Kebnekaise 0 Nulpotjåkka Nulpotjåkka 0 6 (~ n le a
d
a E E E 18 18 18 p a (1810 m) (1810 m)
R Kallaktjåkka Kallaktjåkka (2080 m) (2080 m) Sarektjåhkkå Sarektjåhkkå (1767 m) (1767 m) Skanátjåhkkå Skanátjåhkkå 30ºE Akkajaure Akkajaure (423 - 453 m) (423 - 453 m)
Finland
E E E E 17 17 20ºE 17 N N 68 68 Sweden
10ºE Norway Norway 30’N 30’N
70ºN 65ºN 60ºN 55ºN 67 67 shown in the inset and the black outline indicates the location of Fig 6. Transects of pit excavations and photographs of the field sites are shown in the adjoining panels. Eye symbols Eye panels. adjoining the in shown field sites, and locations referred to inare Fig. theThe2. thesis.map Map locationbedrock of is lithology, the study area in sites the northern Swedish mountains illustrating topography, field the of photographs and excavations pit of Transects 6. Fig of location the indicates outline black the and inset the in shown in the transect panels indicate photographic view points. Relict surfaces and blockfields / northern Swedish mountains 7
To gain greater insight into the development Snow was collected from the accumulation area of a of relict non-glacial surfaces, attention was then nearby glacier, Storglaciären, to test for recent aeolian focussed on the weathering, formation, and ages of additions to the blockfield fine matrix. the autochthonous blockfields that frequently mantle To complement interpretations of blockfield them. Intra-site variations in the weathering of an chemical weathering through analyses of fine amphibolite blockfield were examined on the dry, matrix, numerous randomly selected clasts from clast-dominated summit of Tarfalatjårro (67°55´N, both pits were inspected for evidence of weathering 18°39´E; 1626 m a.s.l.) and in its wet, fine matrix- processes. Surface and subsurface clasts from the rich northern saddle (Paper III; Fig. 2). One pit was summit and saddle pits were inspected for lithology, excavated at each location across a periglacially- roundness, sphericity, pitting, Fe oxidation, and rind sorted circle, from a coarse ring consisting of gravel, development. Summit subsurface clasts were also cobbles, and boulders to a circle centre rich in fine inspected for these differences according to their matrix (i.e. sand, silt, and clay). Fine matrix was location in the wet, matrix-rich sorted circle centre or sampled wherever possible in the coarse rings and at in the dry, clast-dominated ring. 0.2 m depth intervals in the sorted circle centres (Fig. Inter-site variations in autochthonous blockfield 3). Fine matrix samples from the summit and saddle weathering were examined along three altitudinal were then subjected to grain size, X-ray diffraction transects descending from the summits of Alddasčorru (XRD), and X-ray fluorescence (XRF) analyses to (68°25´N, 19°24´E; 1538 m a.s.l.), Duoptečohkka test for variations in chemical weathering according (68°24´N, 19°22´E; 1336 m a.s.l.), and Tarfalatjårro to horizontal position in a sorted circle, depth beneath (Paper IV; Fig. 2). As with Tarfalatjårro, the the surface, and ground wetness. Alddasčorru and Duoptečohkka blockfields are Rock samples were extracted from the summit and also developed in amphibolite. The transects were saddle pits to establish a base for chemical weathering designed to test for weathering differences according of fine matrix through XRF analysis. In addition, to slope position rather than for temperature gradient. granule samples were taken from the summit to test Consequently, altitudinal differences within for a possible chemical weathering link from parent autochthonous blockfields along each transect range bedrock to fine matrix via granular disintegration. from only 73 m for Tarfalatjårro to 88 m and 130 m for
a b CM S S O ORTED RT CIRCLE PERIMETER ED C IRCLE P M M ERIMETER C C
C M
Fig. 3. Blockfield excavations, northern Swedish mountains. a) The summit of Tarfalatjårro. b) The saddle north of Tarfalatjårro. Approximate outlines of the sorted circles into which the pits were excavated are shown by dotted lines and sample locations are illustrated by triangles 8 B.W. Goodfellow / Doctoral Dissertation / 2008
Alddasčorru and Duoptečohkka, respectively. A total Presentation of Papers of 26 pits were hand excavated and blockfield sections examined in 15 pits excavated into coarse sorted circle Paper I rings or into clast-dominated solifluction lobes. Fine matrix samples were taken from 16 blockfield pits for Goodfellow, B.W., 2007. Relict non-glacial surfaces grain size, XRD, and scanning electron microscopy in formerly glaciated landscapes. Earth-Science (SEM) analyses of clay mineralogy. For replication Reviews 80, 47-73. purposes, at least three fine matrix samples were taken from each slope context (i.e. convex summit, straight In this review, I investigate the concept of slope, concave slope base), except for Alddasčorru relict non-glacial surfaces, their characteristics and summit (2 samples) and straight slope (1 sample). preservation within formerly glaciated landscapes, and For comparative purposes, fine matrix samples were the information they convey on long-term landscape taken for grain size, XRD, and SEM analyses from evolution. Because relict surfaces have continued summit till covers on Ruohtahakčorru (68°09´N, to be modified over time, they are polygenetic and 19°20´E; 1349 m a.s.l.) and on Nulpotjåkka (67°48´N, the meaning of relict becomes dependent upon the 18°01´E; 1405 m a.s.l.), and from the high-valley till spatial scale at which a landscape is viewed. Also at the base of the Duoptečohkka transect (1060 m because of continued surface modification during a.s.l.). the Quaternary, non-glacial is preferred to the more The Duoptečohkka and Tarfalatjårro summit pits common term of preglacial when describing relict were excavated where shattered quartz veins were surfaces within formerly glaciated landscapes. visible on the blockfield surfaces. Clasts of vein Relict non-glacial surfaces are frequently quartz were collected from vertical regolith profiles periglaciated upland surfaces that are distinguishable in each pit to determine surface exposure durations from glacial surfaces by large-scale morphologies, from in situ-produced 10Be and 26Al concentrations. including rounded summits, fluvial valleys, and Vein quartz was sampled because fine matrix locally cryoplanation terraces and pediments, and the derived from amphibolite weathering was present in presence of tors, blockfields, and/or saprolites. low quantities and lacking in medium sand (250–500 Blockfields are the most common landform associated µm) and quartz. Although glacial deposition of vein with relict non-glacial surfaces and they remain quartz clasts cannot be entirely excluded, the absence enigmatic features because very few are documented of definite erratics from both summits and the location to be presently forming. They are therefore usually of pits within shattered quartz veins, where the quartz considered to represent periglacially reworked frequently remains attached to amphibolite, make remnants of chemically weathered Neogene regolith it likely the sampled clasts were locally derived profiles. Whereas saprolites have clear Neogene and therefore representative of surface exposure origins, palaeoclimatic interpretations from saprolites histories. are frequently questionable and it is possible that Apparent exposure ages were calculated from a commonly supposed dependence of saprolite nuclide production rates scaled to latitude and altitude formation on a warm, (sub) tropical, climate has been (Stone, 2000) from sea-level high latitude (> 60°) over-estimated. production rates of 4.6 ± 0.3 atoms g-1 yr-1 for 10Be Preservation of relict non-glacial surfaces during (Nishiizumi et al., 2007) and 31.1 ± 1.9 atoms g-1 yr-1 glacial periods occurs primarily through coverage by for 26Al (Lal, 1991). Total surface histories, which non-erosive, cold-based, ice or, in some places, as include periods of burial by ice sheets and changes to nunataks. Glacial features, such as erratics, moraines, nuclide production rates induced by glacial isostasy, boulder blankets, and ice-marginal channels, were then estimated using a 3-dimensional numerical superimposed onto relict non-glacial surfaces ice-sheet model forced by benthic δ18O records indicate former ice covers. TCN analyses of erratics, and incorporating an elastic lithosphere, relaxing tors, and bedrock surfaces are more frequently being asthenosphere (ELRA) model to simulate bedrock used to confirm landscape preservation beneath cold- movement (Bintanja et al., 2002, 2005). based ice. However, the absence of glacial features from relict non-glacial surfaces does not confirm Relict surfaces and blockfields / northern Swedish mountains 9 former nunataks. Whereas features such as sharply evidence of extensive weathering, and the type of defined trimlines have been used to distinguish superficial glacial modification. The surface classes former nunataks from surfaces covered by cold- were verified by fieldwork and their relationships based ice, they might also indicate englacial thermal with elevation, slope gradient, and bedrock lithology boundaries. Confirming former nunataks is difficult explored in the GIS. and the evidence is frequently inconclusive. The study revealed five types of relict non-glacial Where relict non-glacial surfaces have been surfaces: (i) Low activity non-glacial surfaces labelled preglacial, minimal surface lowering and (LA-NG) characterised by a smooth topography morphological change is implied. However, because attributable to creep processes. (ii) Moderate activity of the high capacity for mass wasting under periglacial non-glacial surfaces (MA-NG), characterised by an conditions, it should not be assumed that relict non- uneven topography attributable to solifluction and glacial surfaces are characterised by uniformly low nivation. (iii) High activity non-glacial surfaces (HA- rates of erosion or morphological change. Depending NG), characterised by steep slopes, large quantities of on spatial variables such as bedrock lithology, slope fine matrix, and/or surface instability. Clear breaks in gradient, regolith depth, and the abundance of slope usually mark their boundaries with other surface regolith fine matrix and water, some surfaces, such types. (iv) Non-glacial surfaces with thin, patchy as gently convex summits, are eroding slowly, while regolith covers (PR-NG). (v) Non-glacial surfaces others, such as steep slopes mantled by fine matrix- blanketed by thin (< 2 m) till covers (T-NG). In rich regolith, are likely eroding more rapidly. Spatial addition, two types of extensively weathered glacial variability in denudation rates will produce changes surfaces transitional to relict non-glacial surfaces in relict non-glacial surface morphologies over time. were identified: (i) Glacial erosion (areally scoured) High temporal variability in relict non-glacial surface surfaces (E-G) that appear “roughened” through erosion rates is also likely, with much surface evolution extensive subaerial weathering. (ii) Till-covered having perhaps occurred soon after the initial onset of glacial erosion surfaces (TE-G), characterised by glaciation or during paraglacial phases. The potential allochthonous blockfields (formed from till) mantling for spatially and temporally varying Quaternary glacially eroded bedrock. erosion rates and morphological evolution should be The surface classes display only weak general considered when reconstructing preglacial landscapes trends according to slope and elevation. Because and in subsequent quantifications of glacial erosion. high-elevation surfaces are more likely to be preserved However, much remains to be determined regarding with minimal glacial modification, relict non-glacial relict non-glacial surface erosion rates and the surfaces occur at mean elevations exceeding 1000 m magnitude of morphological changes over time. a.s.l., with LA-NG, MA-NG, and HA-NG surfaces occurring at higher mean elevations (1514–1296 m Paper II a.s.l.) than the glacially modified PR-NG and T-NG surfaces (1200–1018 m a.s.l.). Non-glacial relict Goodfellow, B.W., Stroeven, A.P., Hättestrand, surfaces generally occur on low gradient slopes, with C., Kleman, J., Jansson, K.N., 2008. Deciphering T-NG, PR-NG, and MA-NG surfaces possessing a non-glacial/glacial landscape mosaic in the mean slope angles of 7–14°. LA-NG and HA-NG northern Swedish mountains. Geomorphology 93, surfaces have higher mean slope angles of 19° and 213-232. 21°, respectively, with the higher than expected LA- NG slope gradients attributable to surface armouring Relict non-glacial surfaces in the northern by fine matrix-poor blockfields. Bedrock lithology Swedish mountains were mapped from colour exerts a clear control on surface classes (in part infrared aerial photographs and imposed on a 50 because of its relationship with surface elevation), m DEM in a GIS, where their characteristics were with MA-NG, HA-NG, and, especially, LA-NG analysed. An original feature of this study was the surfaces occurring predominantly on amphibolite, classification of relict non-glacial surfaces according E-G surfaces occurring almost exclusively on to dominant non-glacial surface process regimes, in intrusive lithologies, and T-NG and TE-G surfaces addition to large-scale non-glacial morphologies, mostly occurring on intrusive lithologies and schists, 10 B.W. Goodfellow / Doctoral Dissertation / 2008 respectively. Only PR-NG surfaces exhibit a weak weathering is limited to the production of largely trend with bedrock lithology, being more evenly amorphous Al- and Fe-oxyhydroxides at the summit, distributed across amphibolite, gneiss, schist, and with some additional vermiculitisation and gibbsite intrusive lithologies. crystallisation in the saddle. The variation in fine The presence of different surface classes is matrix chemical weathering between the summit and consistent with the concept of spatial variability saddle may be attributable to slope position, with a in surface processes and rates of non-glacial and greater abundance of fine matrix and water in the glacial landscape evolution. Rather than being saddle, and/or a change in lithology from amphibolite static preglacial remnants, relict non-glacial surfaces on the summit to amphibolite and metapsammite are dynamic features that have continued to evolve in the saddle. Although variations in fine matrix during the Quaternary. The classification provides chemical weathering exist between the summit and hypotheses for landscape evolution that can be saddle, intra-site variations, either with depth or field tested through, for example, TCN studies, across sorted circles, are almost entirely absent from geochemical analysis of fine matrix materials, and both locations. The only, minor, intra-site variation applied to numerical studies of slope development. occurs in the accumulations of sand and granules in The classification may also be applicable to relict narrow spaces between clasts, which appear slightly non-glacial surfaces in other formerly glaciated less chemically weathered than the dominant mixtures landscapes. of sand and silt. In contrast to the fine matrix, more significant Paper III intra-site, as well as inter-site, variations occur in clast chemical weathering. Iron oxidation is more Goodfellow, B.W., Fredin, O., Derron, M.-H., common in summit than in saddle clasts and in the Stroeven, A.P. Weathering processes and wet, matrix-rich sorted circle centres than in the dry, Quaternary origin of an alpine blockfield in Arctic clast-dominated rings. Conversely, surface pitting and Sweden. Manuscript. rind development are relatively more common in the saddle than at the summit and in the dry clast-rich rings The weathering and origin of the Tarfalatjårro at both sites. Inter-site, summit to saddle, variations blockfield, located in the northern Swedish mountains, in clast chemical weathering are attributable to were investigated. To test for weathering variations changes in mineralogy, whereas intra-site variations, according to slope position, clasts and fine matrix across sorted circles, appear to be related to changes were examined from two pits excavated across ridge- in ground moisture. There are additional intra-site top sorted circles; one on a dry summit and the other variations in physical clast weathering, as revealed by in a wet saddle. Fine matrix and clasts were sampled, a general trend of increasing roundness and granular both with depth and across the sorted circles, to also disintegration where clasts are exposed to air, either test for intra-site variations in chemical and physical on the ground surface or in dry, clast-dominated, weathering. Seven fine matrix samples from the parts of the subsurface. Intra-site variations in the summit and eight from the saddle were subjected extent of clast cracking also occur, with cracked clasts to grain size, XRF, and XRD analyses. Clasts were concentrated at the surface and base of the summit inspected for lithology, roundness, sphericity, and pit, but otherwise largely absent. surface weathering features such as Fe oxidation There is no evidence that the Tarfalatjårro and pitting, and split with a hammer and chisel to blockfield is a preglacial weathering remnant. The inspect for internal chemical changes, including rind presence of gibbsite does not indicate advanced development and Fe oxidation. chemical weathering, and both fine matrix and clast Clay quantities are low in all fine matrix samples production can be explained by processes that have (3–6%) and chemical mass balance calculations, operated during the Quaternary. In addition to fine based on Al as the immobile element, revealed matrix bearing little sign of chemical weathering, minor chemical weathering, both at the summit blocks appear to be produced from bedrock through and in the saddle. These findings are supported by pressure release, resulting from the removal of the clay mineralogy, which indicates that chemical overburden, and the opening of existing cracks Relict surfaces and blockfields / northern Swedish mountains 11 through Fe oxidation and annual freeze-thaw cycles exhumed through erosion of overlying regolith, total at the blockfield base. Blockfields are not all the same complex surface histories that are confined to the and may represent Neogene weathering remnants Quaternary do not, in isolation, confirm Quaternary in some locations but in others are the product of blockfield origins. However, taken with the absence processes that have operated during the Quaternary. of significant chemical weathering, a probable Quaternary origin of the blockfields is indicated. Paper IV Present summit blockfield erosion rates are as low as 4.0 mm kyr-1, although higher rates are Goodfellow, B.W., Stroeven, A.P., Fabel, D., Fredin, anticipated in other blockfield-mantled parts of O., Derron, M.-H., Bintanja, R., Caffee, M.W., the landscape, such as those occurring on steeper Freeman, S.P.H.T. Cosmogenic radionuclide slopes, on less resistant lithologies, and/or where fine and fine matrix analyses of blockfields, northern matrix is abundant. Additionally, late Pliocene/early Sweden: Indications of Quaternary origins. Pleistocene erosion rates were also likely to have been Manuscript. much higher, as presumably vegetated regoliths were exposed to increased rates of mass wasting under To further examine blockfield origins and periglacial conditions. However, once fine matrix variations in weathering according to slope has been removed, the blockfields become largely position, blockfields were investigated along three relict features. altitudinal transects descending from the summits of Alddasčorru, Duoptečohkka, and Tarfalatjårro, in Discussion the northern Swedish mountains. Each blockfield is formed on amphibolite bedrock. Quantities and Relict Surface Mapping and GIS Analysis mineral assemblages of clay were determined through grain size and XRD analyses, with additional SEM Geomorphological mapping from colour infrared analysis of fine matrix chemical weathering. Surface aerial photographs revealed that the relict non-glacial exposure durations of blockfields on the summits landscape of the northern Swedish mountains is of Duoptečohkka and Tarfalatjårro were measured composed of a range of surface types (LA-NG, MA- through TCN analyses and incorporated into total NG, HA-NG, PR-NG, and T-NG; Fig. 4). These surface histories that included periods of burial by surface types, which reflect differences in dominant ice sheets, subaerial erosion, and the effects of glacial surface process regimes, are shown, through GIS isostasy on nuclide production rates. analysis, to be weakly related to differences in Fine matrix analyses indicate only incipient elevation and slope gradient. Field surveys also chemical weathering, but which increases in intensity indicate a relationship between surface type and from summits and upper slopes, characterised by the the quantities of regolith fine matrix and water. A production of poorly crystallised oxyhydroxides, stronger relationship is evident between surface type to lower slopes and concave locations, where some and bedrock lithology. additional vermiculitisation and gibbsite crystallisation LA-NG surfaces have a particularly strong occurs. Total complex histories, incorporating correlation with lithology, with > 80% developed exposure, burial, isostasy, and subaerial erosion, on amphibolite and dolerite complexes. These indicate that the acquisition of cosmogenic isotopes hard, sheeted lithologies exert control on surface by present surface regolith is almost certainly, but not characteristics at the regolith scale i.e. they are unequivocally, limited to the Quaternary. weathered and periglacially reworked into thick The investigated blockfields are not remnants blockfield mantles that form the classic “velvet” relict of Neogene deep weathering profiles. Rather, their surfaces (Jansson and Glasser, in press). However, formation can be explained predominantly through because of their resistance to weathering, these physical processes that have operated during the sheeted lithologies also form the highest surfaces in the Quaternary. Because of the possibility that blockfields northern Swedish mountains, which are consequently have formed at depths where they have been shielded isolated from modification during glacial periods, from cosmogenic radiation and gradually been through either protruding above surrounding ice 12 B.W. Goodfellow / Doctoral Dissertation / 2008 % 10ºE 20ºE 30ºE % % 70ºN
65ºN Torneträsk (341 m)
Sweden Finland Norway 60ºN
55ºN
.
.
Akkajaure (423 - 453 m)
.
.
N %
Relict surfaces Low activity non-glacial (LA-NG) surface Moderate activity non-glacial (MA-NG) surface High activity non-glacial (HA-NG) surface Patchy regolith non-glacial (PR-NG) surface Till-covered non-glacial (T-NG) surface Glacial erosion (E-G) surface Till-covered glacial erosion (TE-G) surface
% % % % 0 10 20 30 40 Km
Fig. 4. Map of relict surface classes based on colour infrared aerial photographs. Non-glacial surfaces: LA-NG, MA-NG, and HA- NG, refer to low, moderate, and high activity of surface processes, respectively; PR-NG, Thin, patchy regolith cover; T-NG, Till- covered. Transitional glacial surfaces: E-G, Glacial erosion (areally scoured); TE-G, Till-covered glacial erosion. Map location is shown in the inset (source: Fig. 2 from Paper II, with permission of Elsevier). Relict surfaces and blockfields / northern Swedish mountains 13 sheets as nunataks or being protected beneath cold- RECONSTRUCTED A PREGLACIAL VALLEY ENT SURFA based ice covers. The type of relict non-glacial CURR CE surface occurring at a particular location is therefore directly influenced by bedrock lithological control on GLACIAL EROSION weathering but also indirectly through the presence of particular lithologies at different elevations in the landscape. MISSING B GLACIAL SURFA PRE CE GLACIAL The high surfaces of the northern Swedish EROSION ENT SURFA CURR CE MISSING mountains are frequently illustrated as being composed NON GLACIAL of amphibolite, for example by Kulling (1964), on the EROSION Geological Map, Northern Fennoscandia (1987), on a shapefile geological map available from the Geological Survey of Sweden (2004), and again here in Paper II.
C ACIAL SURFA However, the highest of these surfaces, such as those EGL CE PR OVER ESTIMATE OF GLACIAL on Kebnekaise, are actually composed of dolerite NT SURF CURRE ACE EROSION UNDER ESTIMATE OF NON GLACIAL complexes with minor occurrences of hornfelses EROSION PREGLACIAL SURFACE INCORRECTLY (Andréasson and Gee, 1989). Like amphibolite, these RECONSTRUCTED FROM CURRENT dolerites are hard, coarse-grained, sheeted, mafic SURFACE rocks and no differences in the characteristics of LA- NG or MA-NG surfaces developed on either lithology D IAL SU PREGLACIAL have been observed in aerial photographs or in the GLAC RFACE PRE SURFACE field. Therefore, although references to amphibolite in Paper II should be modified to “amphibolite and SUBSEQUENT TROUGH dolerite,” the inferences based solely on amphibolite SURFACE remain unchanged. The main point derived from the presence of a mosaic of surface types is that the relict non-glacial LOSS OF MATERIAL FROM IAL SU UPPER TROUGH EDGES landscape of the northern Swedish mountains is GLAC RFACE PRE continuing to develop through processes that are spatially variable, both in type and the rates at (! .' SURFACE (! .' SURFACE which they function. Relict non-glacial surfaces are therefore not static preglacial remnants, which has potential implications for reconstructing preglacial IAL SU landscapes and quantifying volumes of glacial erosion GLAC RFACE TRUE PREGLACIAL SURFACE RE ( P ! ' . . ' ! (
Fig. 5. Reconstruction of a local preglacial surface across a MISSING PREGLACIAL SURFACE BASED GLACIAL ON NON GLACIAL PARAGLACIAL glacial trough based on remnant slopes. a). The cross-sectional EROSION SURFACES BORDERING THE area of glacial erosion is calculated by subtracting the current TROUGH topography from the reconstructed topography. b) Under- E PREGLACIAL SURFACE estimation of glacial and non-glacial erosion based on current IAL SU GLAC RFACE non-glacial surfaces and an assumption of static equilibrium. PRE c) An over-estimate of glacial erosion and an under-estimate of VALLEY GLACIER CYCLE non-glacial erosion resulting from evolution of remnant non- glacial surfaces. d) Preglacial surface remnants have undergone a paraglacial response to trough incision, leading to an incorrect preglacial valley reconstruction and an under-estimate of glacial erosion. For simplicity, static equilibrium of remnant preglacial IAL SU PREGLACIAL SURFACE GLAC RFACE surfaces is assumed. e). A preglacial surface erroneously PRE reconstructed from old glacial surfaces bordering the trough. MISSING PREGLACIAL SURFACE Glacial trough development based on Harbor (1992). This GLACIAL INCORRECTLY RECONSTRUCTED EROSION VALLEY GLACIER FROM OLD GLACIAL SURFACES scenario is an addition to those originally depicted in Figure 7 CYCLE BORDERING THE TROUGH from Paper I. 14 B.W. Goodfellow / Doctoral Dissertation / 2008
(Fig. 5). The glacial surfaces that are extensively relict non-glacial surfaces in the Landsat image, (v) weathered, and so transitional to non-glacial surfaces relict non-glacial surfaces blanketed by thin tills at (TE-G, E-G), may, in some locations, predate the last moderate elevations are vegetated, and, (vi) because glaciation and thereby indicate temporal differences in of heterogeneous features in the Landsat image, the ice sheet characteristics from erosive or depositional, textural analysis function in Erdas Imagine identified to passive. surface edges rather than textures. Although future A weakness with the surface classification is that improvements to the model are possible, the human it is a largely qualitative landscape assessment. A eye remains the best tool for identifying relict non- more quantitative approach to surface identification glacial surfaces from aerial or satellite images. from a Landsat image and DEM of part of the study area was attempted through a model developed in Weathering and Origins of the Blockfield Erdas Imagine; a computer program that processes Mantles geospatial raster data. Because relict non-glacial surfaces are generally non-vegetated blockfields, There is no indication of advanced chemical have relatively even surfaces (i.e. amplitudes of weathering in any of the examined blockfields. surface roughness < 3 m), and occur mostly at high Supporting evidence includes the absence of saprolite, elevations and on low gradient slopes, the model soil horizons, and chemically “rotted” clasts from was designed to identify relict non-glacial surfaces all blockfield sections (Fig. 7), minor quantities of according to spectral signatures, surface texture, clay-sized fine matrix, minor formation of secondary elevation, and slope gradient. Landsat bands 2 minerals, the rarity of chemically etched grains in (green), 3 (red), and 4 (near infrared) and a 50 m bulk fine matrix (Fig. 8), and minor elemental losses DEM were used in the analysis. Based on data from in fine matrix compared with parent rock. Clast paper II and experimental analyses in Erdas Imagine, and fine matrix production can be predominantly the following parameters were used for the selection attributed to physical weathering processes that of relict non-glacial surfaces: elevation > 530 m a.s.l., have operated during the Quaternary and there is no slope gradient < 28° and, < 30 m relative relief in a indication of Neogene deep weathering in the origins 3 x 3 pixel moving window. Areas of low spectral of blockfields in the northern Swedish mountains. variation in each of the three bands were also selected Intra-site variations in clast weathering on in ranges that excluded snow, water, vegetation, and Tarfalatjårro occur with depth beneath the ground shadows. surface and across sorted circles. Clasts at the The model results were, however, poor (Table 1; ground surface and in dry parts of the subsurface, Fig. 6), with only small areas of the previously mapped i.e. in the clast-dominated rings of sorted circles, are relict non-glacial surfaces, and some additional subangular, whereas those in the wet subsurface, i.e. glacial surfaces, being identified. This is largely in matrix-dominated sorted circle centres, are very because: (i) the resolutions of the Landsat image angular (Table 5 in Paper III). These trends occur (30 m) and DEM (50 m) are too low, (ii) recently independently of lithology and indicate the operation emerged glacier forefields and high-altitude glacial of granular disintegration at the surface and in the surfaces are also non-vegetated, (iii) the model did not dry subsurface. At these locations, clasts are exposed distinguish elongated forms that frequently indicate to air and therefore subjected to a greater number glacial surfaces from other, non-glacial, surface of, and more rapid, fluctuations in temperature, forms, (iv) snow covered some of the best developed either, or both, of which are important to granular
Table 1. Correlation of relict non-glacial surfaces identified in the model to the input parameters (perfect correlation = 1). Landsat Landsat Band 2 Landsat band 4 Surface Slope band 2 relative band 3 Elevation (near texture gradient (green) difference1 (red) infrared) 0.1330 0.0167 0.1060 0.0961 0.0659 0.1976 0.1084 1Band 2 was selected because it provided the best correlation with relict non-glacial surfaces mapped in Paper II. Areas of low spectral variation (i.e. low relative difference) were selected in an attempt to identify blockfields. Relict surfaces and blockfields / northern Swedish mountains 15
Legend Relict non-glacial surfaces (mapped from aerial photographs)
Relict non-glacial surfaces (modelled selection, 50 x 50 m pixels) Hydrology
0 25 km
Fig. 6. Model output for automated selection of relict non-glacial surfaces (white) compared with the mapped surface distribution (brown). Figure location shown in Fig. 2. disintegration (White, 1976; Matsuoka, 1990; chemically weathered fine matrix on the Tarfalatjårro Ballantyne and Harris, 1994, pp. 164-165). The role summit had accumulated between horizontally of ground moisture in weathering is highlighted by aligned slabs of amphibolite (samples BG-06-23 and the inhibition of Fe oxidation in the dry subsurface BG-06-24 in Tables 1 and 4, Paper III), and the least clasts (Table 6 in Paper III). Intra-site differences in chemically weathered fine matrix in the Tarfalatjårro weathering indicate the long-term stability of these saddle had accumulated beneath dry gravel in the periglacially-sorted circles. coarse sorted circle ring (sample BG-06-07 in Table Some additional, minor, intra-site variations also 4, Paper III). However, soil horizons were absent occur in fine matrix weathering. The coarsest and least from both locations (Fig. 9) and there are no intra-site 16 B.W. Goodfellow / Doctoral Dissertation / 2008
(a) 0
-0.3
-0.7
-1.3 (m)
0 0.25 0.50 0.75 1.00 1.25 1.50 (m) (b) 0 -1 -2 bedrock surface -3 -4 -5 (m) 200 220 240 260 280 300 320 340 360 380 (m)
Fig. 7. A representative blockfield section based from data from 17 pits excavated on Alddasčorru, Duoptečohkka, and Tarfalatjårro, and a ground penetrating radar image of bedrock beneath a till sheet on Ruohtahakčorru. a). A coarse surface layer dominated by boulders and cobbles is underlain by a gravely layer, which extends to a depth of about 0.7 m. Below this occurs a layer of boulders imbedded in a silty-sandy matrix to a depth of 1–1.3 m. The bottom of the blockfield is marked by a layer of large slabs of fractured bedrock, which continue to a depth of 2–4 m, where solid bedrock is reached (Isaksen et al., 2001). Chemically “rotted” clasts, soil horizons, and saprolite are absent from all examined blockfields. b). Ground penetrating radar was used in an attempt to determine depths to bedrock beneath blockfield mantles. Distinct bedrock surfaces were not observed, as illustrated at the left of the figure, except for this section through the till-covered summit of Ruohtahakčorru, where a bedrock surface was observed beneath 2–2.3 m of clast-rich, weathered, till. Relict surfaces and blockfields / northern Swedish mountains 17
A B
20 μm 20 μm
C D
%PIDOTE
!LBITE
Fig. 8. Scanning electron microscope images indicating slight chemical weathering of fine matrix. a) Albite, with a chemically etched surface; the only etched grain identified. b) Chemically unaltered amphibole, typical of all SEM images of amphibole. c) Disintegrating albite, possibly through chemical processes. d) Chemically unaltered epidote, typical of all SEM images of epidote. variations in the incipient development of secondary changes in chemical weathering between the summit minerals (Table 1 in Paper III). Homogenisation of and saddle (Table 4 in Paper III), although a real the fine matrix through recent periglacial churning change may be obscured by small sample sizes is discounted by the intra-site differences in clast and variability in bedrock mineralogy. Whereas weathering. These data therefore reinforce the there are no statistically significant changes in clay interpretation of minimal chemical weathering of this volumes along the transects, the Tarfalatjårro saddle blockfield. is significantly less sandy than the summit (at 1σ), Inter-site variations in blockfield weathering where clast oxidation is also more prevalent (Tables are evident along the three altitudinal transects 1, 5 and 6 in Paper III). (Alddasčorru, Duoptečohkka, Tarfalatjårro). The Because of varying mineral proportions in primary difference occurs in clay mineralogy, where amphibolite between the Tarfalatjårro summit and chemical weathering on summits is limited to the saddle, and the additional presence of metapsammite production of poorly crystallised oxyhydroxides but in the saddle (Table 5 in Paper III), it is impossible increases downslope to be relatively most advanced to distinguish variations in chemical weathering in concave locations, where some vermiculitisation attributable to slope position from those attributable and gibbsite crystallisation occurs (Table 1 in Paper to mineralogy. However, supplementary evidence III and Table 2 in Paper IV). Isocon plots of mobile from the more homogenous amphibolite blockfields and immobile elements also indicate slight chemical of Alddasčorru and Duoptečohkka, where the weathering in the Tarfalatjårro saddle, whereas almost occurrences of secondary minerals also increase none occurs at the summit (Fig. 10). However, mass downslope (Table 2 in Paper IV), indicates that balance calculations reveal no statistically significant slope position is important to blockfield chemical 18 B.W. Goodfellow / Doctoral Dissertation / 2008
0 20
2 samples
-0.2 Summit fine matrix t) fi t Summit rock s e Summit granules 16 P2O5 f b o e -0.4 Saddle fine matrix lin ( n o Saddle rock c o is t n e m Al2O3 -0.6 le e ile 12 b o m MnO m -0.8 Sr I TiO 2 V Regolith depth (m) Fe O -1.0 2 3 8 Y Th 2 samples Cu
-1.2 SiO2 Cr Zn Zr 4 Na2O -1.4 BG-06-17 (fine matrix, 1.1 m depth, circle edge) CaO Legend K O MgO 36 38 40 42 44 46 48 50 52 54 56 2 + Immobile element used to contruct isocon Ga Chemical Index of Alteration (CIA) + ‘Immobile’ element excluded from isocon Nb Co Ni o Mobile element Ba Fig. 9. Chemical Index of Alteration (CIA) of bulk fine matrix 0 with regolith depth, Tarfalatjårro summit and saddle. The CIA 0 4 8 12 16 20 is a dimensionless index (0–100) that assumes immobility of BG-06-31 (rock from pit wall) Al in weathering mantles. Values > 65 indicate the presence 20 K O of secondary minerals. The absence of significant variations 2 P2O5 in fine matrix with regolith depth highlights an absence of soil V profiles. Sr 16 MnO Al2O3 t) fi TiO t 2 weathering. An increase in regolith water quantities s e b f o e and/or longer residence times of fine matrix that lin 12 ( n o Cr c o has been transported downslope may be the most is t en m Fe2O3 le important factors contributing to the increased e ile Zn b o chemical weathering of blockfields on lower m 8 Im slopes and in concave locations. Small altitudinal Cu SiO differences within autochthonous blockfields along Ba 2 CaO MgO each transect likely exclude weathering variations BG-06-05 (fine matrix, surface, circle edge) 4 Ni Y Legend according to temperature. Minor chemical weathering Co + Immobile element used to contruct isocon Zr Na O 2 + ‘Immobile’ element excluded from isocon Ga in all locations, including similar secondary mineral o Mobile element Nb formation in tills (Table 2 in Paper IV), and clay 0 abundances that are at the low end of the range 0 4 8 12 16 20 previously reported for other blockfields (Caine, BG-06-34 (rock from pit wall) 1968; Ahnert, 1977; Rea et al., 1996a,b; Dredge, Fig. 10. Isocon plots of fine matrix versus rock from the summit 2000; Marquette et al., 2004; Paasche et al., 2006), (top) and saddle (bottom) of Tarfalatjårro. Element concentrations support likely Quaternary origins for these blockfields. have been scaled by the same factor in each sample to fit on the However, the lower quantities of clay and rarity of plots and the isocon is the line of best fit through the immobile elements (black crosses). Elements marked by grey crosses are gibbsite in till compared with blockfield fine matrix sometimes immobile and used in isocon calculations. However, indicates that these blockfields have formed over they are excluded here because they have been either mobilised multiple glacial-interglacial cycles. in the regolith or vary significantly in concentration between Fine matrix chemistry and clay mineralogy have rock samples from within each site. The scaled concentrations been used only semi-quantitatively in this analysis of of V and Cr in the saddle parent rock sample are 30 and 21.8, respectively. blockfield chemical weathering. Further quantitative analyses of fine matrix chemistry, such as comparing fine matrix. Quantitative analysis of clay minerals the strains of the fine matrix samples, were not was not attempted because of the associated technical undertaken because of the natural variability of the difficulties (Moore and Reynolds, 1997, p. 298). parent bedrock and the minor elemental losses in the Peak intensities in soils are uncertain and affected Relict surfaces and blockfields / northern Swedish mountains 19 by the concentrations of Mg and Fe. Quantitative northern Swedish mountains can be explained by analysis of mixtures of clay and non-clay minerals processes that have operated during the Quaternary. also requires the preparation of non-oriented samples (i.e. mineral flakes are not oriented parallel with Proposed Model of Blockfield Formation the substrate; Moore and Reynolds, 1997, p. 220), whereas these samples were oriented. In addition, Subsurface weathering processes are crucial in errors in quantitative analysis are much larger if the the proposed model of blockfield formation (Fig. 11). proportions of clay minerals in a sample are low Pressure release, through the removal of overburden, (Moore and Reynolds, 1997, p. 299). Because these cracks and fragments bedrock at the regolith base. samples indicated incipient chemical weathering, Whereas permafrost protects the deepest regolith there was little incentive to attempt quantitative clay (> 1.6 m) from further weathering, its impermeable mineral analysis even if technical issues could be surface ensures that the blockfield profile near the overcome. base of the active layer is saturated with water during The summit blockfields of Duoptečohkka and, warmer months. Ideal conditions are therefore particularly, Tarfalatjårro are eroding slowly. From provided for further crack growth and eventual rock 10Be concentrations in blockfield profiles and splitting through annual freeze-thaw cycles (Hallet modelled periods of surface burial by ice sheets et al., 1991; Murton, 1996; Murton et al., 2000; and glacial isostasy, maximum long-term subaerial Matsuoka, 2001). Oxidising Fe along mineral planes erosion rates are calculated to be 6.8 mm kyr-1 and 4.0 may also promote crack growth by weakening the mm kyr-1, respectively. Whereas subaerial erosion rock structure and providing an entry point for water. of the Tarfalatjårro summit is likely to be relatively The absence of saturated conditions at the ground constant through fine matrix removal, the presently surface may explain why frost wedging has rarely thin regolith cover indicates possible regolith plucking been observed in the field (Ballantyne and Harris, by over-riding ice sheets and, therefore, a potential 1994, p. 164; Matsuoka, 2001). over-estimation of the Duoptečohkka maximum Clasts are eventually exposed at the surface through subaerial erosion rate. Because the blockfield forms regolith stripping and periglacial sorting during cold, a thick (1.3 m), fine matrix-poor, mantle on a gently but ice sheet free, periods. Once at the surface, or convex summit, the Tarfalatjårro summit erosion rate deposited through circular sediment movement into is likely to be near the minimum for relict non-glacial the dry clast-dominated parts of periglacially-sorted surfaces in this landscape. circles or stripes (Hallet and Prestrud, 1986; Ballantyne Various surface history scenarios modelled from and Harris, 1994, p. 93), clasts are rounded through 10Be and 26Al concentrations indicate that the exposure granular disintegration. On summits, fine matrix is of present blockfield regolith to cosmogenic radiation redistributed by periglacial sorting into circle centres is limited to the Quaternary (Table 2). Although and/or winnowed out by water flowing through the surface histories, which incorporate periods of surface profile, whereas it accumulates downslope in concave burial by ice sheets, changes in nuclide production locations. The rate of blockfield formation is likely rates attributable to glacial isostasy, and maximum controlled by the rate of surface denudation and active surface erosion rates permitted by the ice sheet model layer depth, where a slow denudation rate, resulting (at the erosion rate resolutions given in Table 2), are perhaps from the winnowing out of fine matrix confined to the Quaternary, it remains possible that (Boelhouwers, 2004) inhibits blockfield development. present surface regolith formed at a depth where it Whereas a deep active layer exposes more bedrock was shielded from cosmogenic radiation and has to weathering, a shallow active layer concentrates been gradually revealed through erosion of overlying weathering and erosion into a shallower near-surface regolith. Consequently, a Neogene blockfield origin zone. This likely results in more vigorous periglacial cannot be excluded through TCN analyses alone. mass wasting and downwards translocation of the However, when total surface histories are considered weathering front through the blockfield, thereby with the various evidence for minor chemical enhancing the rate of blockfield formation. The weathering, there is no reason to appeal to a Neogene operation of processes most important to blockfield origin for blockfields. Blockfield formation in the formation at depth within an existing regolith cover, 20 B.W. Goodfellow / Doctoral Dissertation / 2008
Table 2. Complex history model results for unmixed Duoptečohkka and Tarfalatjårro vertical blockfield profiles. The zero erosion and zero isostasy scenarios provide minimum total histories for each site, whereas the scenarios including maximum erosion rates permitted by the complex history models provide maximum total histories, at the given erosion rate resolutions.
[10Be] atom Exposure prior Exposure since Shielding Total history Site Scenario1 g-1 to shielding deglaciation (103 yr) (103 yr) (x 105) (103 yr) (103 yr)
0 erosion Duoptečohkka 1.53 6.7 65.0 10.6 99.5 0 isostasy 0 erosion Duoptečohkka 1.53 0.4 71.9 10.6 109.9 Isostasy 16.40057 Duoptečohkka mm ka-1 1.53 12.8 506.3 10.6 1020.1 0 isostasy 15.11518 Duoptečohkka mm ka-1 1.53 1.2 187.9 10.6 282.2 0 isostasy 15.11518 Duoptečohkka mm ka-1 1.53 27.5 525.8 10.6 1070.8 Isostasy 0 erosion Tarfalatjårro 3.67 4.0 82.3 11.8 166.3 0 isostasy 0 erosion Tarfalatjårro 3.67 10.6 82.3 11.8 172.9 Isostasy
6.64 mm ka-1 Tarfalatjårro 3.67 55.4 292.6 11.8 862.1 0 isostasy
6.22 mm ka-1 Tarfalatjårro 3.67 34.9 150.8 11.8 394.2 0 isostasy
6.22 mm ka-1 Tarfalatjårro 3.67 7.3 344.6 11.8 979.7 Isostasy
1Erosion rates were determined iteratively from the complex history models for Duoptečohkka and Tarfalatjårro. The high erosion rate precisions are necessary because of the exponential increase in total surface histories as the maximum erosion rates permitted by the complex history models are approached but never reached. The 15.11518 mm kyr-1 erosion rate is the maximum permitted, at 10-5 resolution, by the Duoptečohkka complex history model when glacial isostasy in included. Maximum erosion rates, at 10-2 resolution, permitted by the Tarfalatjårro complex history model are 6.22 mm kyr-1, with isostasy, and 6.64 mm kyr-1, without isostasy. Total surface histories for Tarfalatjårro based on the same erosion rate precision as for Duoptečohkka could not be calculated because they exceeded the time span (1.07 Myr) covered by the complex history models. Total surface histories continue to increase exponentially with further refinements in erosion rate precision. rather than at the surface, may explain why so few to be the primary source of blockfield fine matrix. blockfields seem to be presently forming. Silt production through frost weathering has been The fine matrix present in blockfields appears observed in laboratory experiments (Lautridou and to be a product of in situ weathering. There is no Ozouf, 1982; Smith et al., 2002) and the development evidence from fine matrix chemistry (Table 5, Fig. of relict non-glacial surface regolith over multiple 10), or from grain shapes revealed by SEM (Fig. 8) glacial-interglacial cycles apparently offers sufficient and light microscopy (Fig. 12), for the presence of time for frost weathering to produce substantial aeolian or glacial sediments. Additional evidence for quantities of fine matrix. Erosive processes, such as the lack of aeolian sediments includes the absence frost creep and surface and subsurface wash, deplete of ventifacted boulders, clear aeolian deposits, and convex summit areas of fine matrix and concentrate current aeolian dust transport as revealed by the it on slopes and in concave slope bases. presence only of dried lichens in fines collected from Chemical weathering is a minor contributor to fine the accumulation area of Storglaciären. matrix development. Chemical data (Table 4 in Paper Physical breakdown of rock through, for III) indicate that granules have the same composition example, frost weathering (microgelivation), appears as parent rock and are not an intermediate step in Relict surfaces and blockfields / northern Swedish mountains 21
5. Rounding, through granular disintegration, of clasts exposed to air at 4. Formation of clast-rich sorted circle rings and fine matrix-rich circle the ground surface and in coarse sorted circle rings. Granules accumulate interiors through circulatory sediment displacement. Because of low between clasts just below the surface in the sorted circle interiors and towards relative volumes of fine matrix, sorting is incomplete, with clasts the bases of the coarse rings. remaining in circle centres, aligned horizontally, and concentrated near the surface. 0 6. Frost cracking of some surface clasts. 0.1
0.2
0.3 C la nt st e d em is c p la 0.4 la p c is e d m t e s 0.5 a n l t C 0.6
F
i t n
n e 0.7 e
m m a e t c r a 0.8 ix l p d is is p d la rix 0.9 c at em m e e nt Fin 1.0
Depth (m) 1.1
1.2
1.3 2. Further crack growth through macrogelivation 3. Fine matrix production through microgelivation in saturated rock, induced by ice segregation 1.4 resulting primarily from ice segregation. near the base of the active layer and in the upper 1.5 permafrost. Fe oxidation may also assist crack Present maximum annual depth of the active layer (approximate) development. 1.6
1.7 1. Bedrock cracking through pressure release. 1.8
1.9
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 Horizontal Distance (m)
Fig. 11. Model for autochthonous blockfield formation in the northern Swedish mountains. The illustration is based on the Tarfalatjårro summit and saddle excavations, notations indicate the main elements of the model described in the text, and the active layer depth is based on data from a permafrost monitoring station in the Tarfalatjårro saddle (Fig. 2; Isaksen et al., 2007). fine matrix production through chemical weathering. to landscape-scale topographic evolution of these The additional absence of chemically “rotted” clasts surfaces and placing them in the context of similar and saprolite, and possibly the increase in chemical landscapes in other periglaciated and/or formerly weathering away from summits, also indicate that glaciated locations. most chemical alteration of fine matrix probably For example, TCN and numerical analyses of some occurs after it has been produced. similar blockfield-mantled, relict non-glacial surfaces in the Rocky Mountains indicate the presence of Future Research convex summit areas with slope gradients and regolith transport rates that increase linearly away from the This study has focussed, firstly, on identifying summit apexes (Small et al., 1999; Anderson, 2002). and characterising relict non-glacial surfaces in the Also because regolith thicknesses and production northern Swedish mountains and, secondly, analysing rates are constant downslope, it was concluded that the weathering and origins of the blockfields that these surfaces are lowering in a state of dynamic frequently mantle them. Key future research equilibrium. However, total station surveys indicate directions lie in determining the long-term hillslope that relict non-glacial surfaces in the northern Swedish 22 B.W. Goodfellow / Doctoral Dissertation / 2008
allow for better preglacial surface reconstructions, with subsequent improved quantifications of glacial erosion, and contribute to general long-term, large- scale landscape evolution theory. Work is also needed to further quantify differences in the non-glacial and transitional surface categories identified in this study and to expand these findings to other formerly glaciated regions. At present, erosion rates of only two LA-NG surfaces have been measured. Erosion rates or surface exposure histories for MA-NG, HA-NG, PR-NG, TE-G, E-G surfaces remain unknown apart from one 10Be measurement of an E-G surface, which provided an erosion rate of 23.6 ± 1.9 mm kyr-1 or an apparent exposure age of 23.0 ± 1.8 kyr, indicating that it was not significantly eroded during the last glaciation. TCN analyses may be applied to MA-NG summit regolith profiles, and to bedrock on PR-NG, TE-G, and E-G surfaces. However, they could not be effectively applied to HA-NG surfaces, which occur on straight slopes away from summits, unless quartz bearing bedrock could be reached beneath a shallow regolith cover that was of sufficient stability to be safely excavated. Similar mapping of relict non-glacial surfaces could be attempted in other formerly glaciated landscapes to test the general applicability of this surface classification. An additional benefit of such an exercise may be that the surface categories could be Fig. 12. Optical microscope images of fine matrix from the 10 26 Tarfalatjårro summit and saddle blockfields. A). Sample BG- examined through Be and Al measurements on 06-12 from 0.90 m depth, Tarfalatjårro saddle. B). Sample lithologies containing more quartz than the generally BG-06-22 from the surface, Tarfalatjårro summit. Amphibole, mafic rocks occurring on the high surfaces of the muscovite, feldspar, and quartz are visible in both images. northern Swedish mountains. However, well-rounded grains are absent (Photographs: Ola Surface weathering could be tested along Fredin, Geological Survey of Norway, Trondheim). altitudinal transects that extend to lower elevations mountains are comprised of straight slopes, with only than in the present study, with recent glacial sediments small convex summit areas (Fig. 13). These surfaces being examined to better understand rates of chemical therefore have similar elements to hillslopes in steep, weathering under recent environmental conditions. rapidly eroding catchments, such as in the Oregon Spring water should also be tested for dissolution coastal ranges (Roering et al., 1999), but with lower products to assess current chemical weathering. slope gradients. The straight slopes exhibit a nonlinear The proposed mechanism of blockfield formation relationship between gradient and regolith transport through subsurface frost weathering also requires rate, as opposed to the convex summits, which verification through process studies. Although exhibit a linear relationship. Dynamic equilibrium, technically challenging, field measurements of with respect to surface lowering, is not indicated in subsurface temperatures are required. In addition, this landscape. Although slope retreat may still exist laboratory and numerical studies, similar to those in a state of dynamic equilibrium, the efficacy of that have been performed on bedrock (Hallet et al., processes such as solifluction and nivation make it 1991; Anderson, 1998; Murton et al., 2000), could be unlikely (Font et al., 2006). A better understanding designed to assess the role of frost weathering beneath of the topographic evolution of these surfaces will blockfield mantles. Such studies would have general Relict surfaces and blockfields / northern Swedish mountains 23
(a) (b) 1630 1350 1300 1590 1250 1550 1200 Elevation (m a.s.l.) 1510 Elevation (m a.s.l.) 1150 0 250 500 750 1000 1250 1500 0 500 1000 1500 2000 2500 Horizontal distance (m) Horizontal distance (m) 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0.0 0 -0.1
Tangent of slope angle 0 250 500 750 1000 1250 1500 Tangent of slope angle 0 500 1000 1500 2000 2500 Horizontal distance (m) Horizontal distance (m)
(c) (d) 1630 1360
1580 1340 1530 1320 1480
Elevation (m a.s.l.) 1430 Elevation (m a.s.l.) 1300 0 100 200 300 400 500 600 700 0 100 200 300 400 500 Horizontal distance (m) Horizontal distance (m) 0.5 0.8
0.4 0.6 0.3 0.4 0.2 0.1 0.2 0 0 Tangent of slope angle Tangent of slope angle 0 100 200 300 400 500 600 700 0 100 200 300 400 500 Horizontal distance (m) Horizontal distance (m)
Fig. 13. Total station surveys of the Tarfalatjårro (left) and Ruohtahakčorru (right) summits. a). Tarfalatjårro long profile. b). Ruohtahakčorru long profile. c). Tarfalatjårro cross profile. d). Ruohtahakčorru cross profile. The tangents of the slope angles are shown below each profile. Convex (i.e. tangents of slope angles increase linearly away from the summit crest) and straight slope segments for each profile are illustrated by short dash and long dash lines, respectively. importance in providing insight into a mechanism for have preglacial origins. In addition to illustrating a bedrock conversion to regolith. spatial scale dependency for the term “relict” (Fig. Although the blockfields of the northern Swedish 14), the extent to which the present “non-glacial” mountains have Quaternary origins, the applicability landscape represents a “preglacial” landscape is a of the findings of this study to other blockfields remains question that also requires further exploration. unknown. It is likely that, unless large quantities of secondary minerals are present, other blockfields are Conclusions also the product of weathering processes that have operated during the Quaternary. However, while Many high elevation surfaces in the northern the blockfields of northern Sweden have Quaternary Swedish mountains exhibit relict non-glacial origins, the surfaces they mantle have not been characteristics. However, considerable spatial significantly modified through glacial processes and variation occurs, with five classes of relict non-glacial 24 B.W. Goodfellow / Doctoral Dissertation / 2008
(OLOCENE SURFACE FEATURES
SO LIFLU S CTION SORTED STRIPE LOBES
SORTED CIRCLES
1UATERNARY REGOLITH
LARGE SCALE MORPHOLOGY WITH PRE GLACIAL ORIGINS
Fig 14. Relict non-glacial surface with a polygenetic history. Large-scale morphology with preglacial origins is mantled by Quaternary regolith, with imprints of Holocene geomorphology (source: Paper I, with permission of Elsevier). surfaces being identifiable from colour infrared aerial sorted circle centres. Chemical weathering of fine photographs. The surface classes reflect differences matrix is highly restricted on summits, where only in the types and/or rates of weathering and erosional poorly crystallised oxyhydroxides are produced, but processes acting upon them, according to elevation, increases slightly in concave locations, where some slope gradients, volumes of regolith fine matrix and additional vermiculisation and gibbsite crystallisation water, the style and degree of superficial glacial occurs. There is no evidence for aeolian and/or glacial modification, and, particularly, bedrock lithology. A additions of fine matrix, which appears primarily to further two classes of extensively weathered glacial be a product of in situ frost weathering. surfaces highlight temporal variations in glacial Subsurface, rather than surface, weathering processes and potentially provide information on processes may be most important to blockfield rates of subaerial weathering. Because of spatially formation. Regolith covers maintain water in variable process rates, it should not be assumed that contact with bedrock surfaces and (annual cycle) relict non-glacial surfaces at the same elevations are frost weathering may be most effective in the necessarily remnants of once-widespread preglacial water-saturated bases of blockfield profiles, above erosion surfaces. impermeable permafrost surfaces. The operation of The weathering of autochthonous blockfields, the processes most important to blockfield formation which commonly mantle relict non-glacial surfaces, at depth within an existing regolith cover, rather than is dominated by physical processes. Bedrock, at the surface or on subaerially exposed bedrock, may composed of sheeted lithologies such as amphibolite, explain why so few blockfields appear, from surface is fractured through pressure release, with further characteristics, to be presently forming. splitting potentially occurring through Fe oxidation There has been a heavy reliance on the presence and frost weathering, on a seasonal basis, of rock of particular secondary minerals, such as gibbsite and located in the water-saturated zone at the base of kaolinite, in determining the origins of blockfields the active layer. Clasts at the surface and in the dry, or other palaeoregoliths in formerly glaciated coarse rings of sorted circles are also weathered landscapes (Rea et al., 1996b; Ballantyne, 1998; through granular disintegration, with chemical Ballantyne et al., 1998; Paasche et al., 2006; André changes restricted to occasional rind development et al., 2008). It is the abundance of these minerals in these dry locations and frequent Fe oxidation in that is important and their presence in a mixture wet locations, such as the fine matrix-dominated dominated by primary minerals is not diagnostic of Relict surfaces and blockfields / northern Swedish mountains 25 regolith formation under a warm, preglacial climate. (near!) PhD completion. Foremost, I wish to thank The presence, in this study, of gibbsite in till (Table 2 my sister, Tara Goodfellow, who has had to bear much in Paper IV) and its association with otherwise poorly of our family responsibility while I have been away crystallised oxyhydroxides indicates that it does from Australia. She has done this uncomplainingly not represent advanced chemical weathering in this and with admirable dedication, and my PhD would setting. Chemical weathering of blockfields in the have been simply impossible without her efforts. northern Swedish mountains is of similar intensity to Secondly, I wish to thank my primary supervisor, chemical weathering of Quaternary regoliths in other Arjen Stroeven, for his support and encouragement alpine settings. To best define the extent of chemical over the past 4 years. Arjen has been on fieldwork weathering in palaeoregoliths, such as blockfields, with me, supported me financially, meticulously fine matrix chemistry should be examined in addition reviewed manuscripts, discussed scientific issues, to its mineralogy. and provided enthusiasm and encouragement. I The summits of Duoptečohkka and Tarfalatjårro am greatly indebted to his efforts. I also wish to are eroding slowly, at maximum modelled rates of acknowledge my co-supervisors, Clas Hättestrand, 6.8 mm kyr-1 and 4.0 mm kyr-1, respectively. The Johan Kleman, and Krister Jansson, who have also Tarfalatjårro erosion rate is likely near the low end been enthusiastic, supportive, and generous with their of the range for relict non-glacial surfaces in this time during supervisor meetings and whenever I have landscape because it is measured at the apex of a approached them with various issues. I have also been gently convex summit, mantled by a fine matrix-poor privileged to have done fieldwork with each of them blockfield. Complex exposure histories from TCN and to have had them review my manuscripts. While profiles, incorporating periods of surface burial by ice “Red Pen” Stroeven is a well-known phenomenon, sheets and the effects of glacial isostasy, are almost Clas has also become very liberal in his application certainly confined to the Quaternary on both summits of the dreaded red pen! (1070.8 kyr for Duoptečohkka, at 10-5 erosion rate A number of people have contributed their time resolution, and 979.7 kyr for Tarfalatjårro, at 10-2 and skills in laboratory work and in discussing erosion rate resolution). analyses with me. Derek Fabel (University of There is no need to appeal to Neogene weathering Glasgow) processed a number of my samples for processes to explain the presence of blockfields in cosmogenic isotope analyses, assisted in teaching me the northern Swedish mountains. All observations of these analyses, allowed me to work for a month in his these blockfields can be explained by processes that state-of-the-art laboratory at SUERC, East Kilbride, have operated during the Quaternary. Whereas relict processed raw AMS data, and generously discussed non-glacial remnants have preglacial origins, their results with me. Cosmogenics is a complicated blockfield mantles do not. A polygenetic landscape business and Derek has patiently answered my history is therefore indicated, where the potential age various questions. In addition, I have on a couple of the landscape increases with the spatial scale at of occasions stayed with Derek and am grateful to which it is viewed. both him and his wife, Mandy, for their hospitality. Unlike non-glacial surface remnants in some other Furthermore, it has been enormously enjoyable locations, those in the northern Swedish mountains working with a fellow Australian while so far from are characterised by small convex summits and home. straight slopes. Dynamic equilibrium is therefore Ola Fredin spent a week with me in a tent on the not indicated in this landscape and there remains Tjeurolako plateau and helped with the unimaginably considerable scope to link regolith weathering and exciting (ahem…!) work of excavating blockfield erosion rate studies with hillslope- and regional-scale pits on Tarfala ridge. Subsequently, Ola generously development of relict non-glacial surfaces. arranged for me to do soil chemistry analyses at the Geological Survey of Norway (NGU) in Trondheim. Acknowledgements I cannot overstate how important these analyses have been to my PhD or my debt to Ola for providing Numerous people have contributed enormously me with this opportunity. I also thank Ola and his over the last few years to getting me to this point of wife, Lena, for their hospitality during my first trip to 26 B.W. Goodfellow / Doctoral Dissertation / 2008
Trondheim, with an additional thanks to Lena for her Glasser (University of Wales) for his advice and good advice on various issues, and for her upfront criticism humour on issues ranging from Krister’s cooking to of the poor state of my Swedish language skills! my AGU 2007 oral presentation and job hunting. My While working at NGU, I’ve also had the great postgraduate diploma supervisor, Wayne Stephenson pleasure of learning how to interpret geochemical (University of Melbourne), is thanked for giving me data from Marc-Henri Derron. Marc and I have spent the start that ultimately led down this path. many days interpreting grain size, XRD, XRF, and Research is impossible without money so I SEM data, and while staying with him in Trondheim thank the following funding sources: the Swedish he also introduced me to the fine Norwegian-Swiss Society for Anthropology and Geography (SSAG) combination of reindeer fondue! I therefore also Andréefonden, Axel Lagrelius’ fond, C.F. Liljevalch thank Marc for his hospitality and culinary skills. J:ors resestipendium, K & A Wallenbergs Stiftelse, Without Derek, Ola, and Marc, my PhD would have Lars Hiertas minne Stiftelse, Carl Mannerfelts fond, been greatly diminished and, unquestionably, a lot Ahlmanns fond, and the U.S. National Science less fun! Foundation (NSF). I wish also to acknowledge Maria Miguens- Research is also impossible without stimulating Rodriguez (SUERC) for guiding me through the ideas so I wish to acknowledge two papers that labour-intensive processes of sample preparation inspired the initiation of this project: Small et al., for AMS. Maria’s laboratory habits are first class 1999. Estimates of the rate of regolith production and when dealing with expensive equipment and using 10Be and 26Al from an alpine hillslope. deadly chemicals including large volumes of hot, Geomorphology 27, 131-150; and Anderson, 2002. concentrated, hydrofluoric acid and the equally Modeling the tor-dotted crests, bedrock edges, and lethal beryllium oxide, her fastidious methods are parabolic profiles of high alpine surfaces of Wind reassuring. I am grateful to have learned a large part River Range, Wyoming. Geomorphology 46, 25-58. of AMS sample preparation from her. Although my project evolved into a study different I also thank my field assistants and good mates, Ulf from these two, they provided the initial “hook”. Jonsell, Ben “Legs” Kirk, Damian “Waldo” Waldron, While in Sweden I have been privileged to have and Mark “Ol’ Man” Wareing for their efforts in pit made many good friends at INK, from my Swedish excavation and in carrying equipment and rocks up language classes (we didn’t learn much Swedish and down steep, unstable, and frequently wet, slopes. but we had a lot of fun…and drank a lot of beer In particular, I thank Legs and Waldo for enduring, afterwards), through the Australian connection at without complaint, 10 days of rain, snow, cold, my Karolinska Institutet, and especially from the Aussie camp cooking, sharing a tent with each other, the Rules Footy club. Life experience is best measured world’s crankiest helicopter proprietor, and general in those you meet and the laughs you have over a misery; conditions to which Australians are poorly beer or on a footy trip, and it is fair to say my time adapted. I am also grateful that you have forgiven in Stockholm would have been immensely poorer me. without all of these people. Thanks a lot! I gratefully acknowledge the following people who As good as it is, completing a PhD will be the have contributed their various expertises to my PhD second, and soon the third, best thing that has happened studies: Göran Alm (INK), Eve Arnold (IGG), Karna to me since I have been in Stockholm. Easily the best Lidmar-Bergström (INK), Richard Bintanja (Royal thing has been meeting my partner, Hanna Sjölin, Netherlands Meteorological Institute), Ian Brown who has provided me with encouragement, laughs, (INK), Marc Caffee (Purdue University), Jon Harbor friendship, a roof over my head, and lots of love. (Purdue University), David Jarman, Joakim Mansfeld Equally best will be the birth of our first child in (IGG), Maj-Liz Norberg (INK), Vicky Pease (IGG), the summer. I look forward to a happy and exciting Per-Olof Persson (Natural History Museum), Alasdair future with both of you! Skelton (IGG), Colin Thorn (University of Illinois), Finally, I dedicate this work to my mother, Lynette and the staff at Tarfala Research Station. I thank Goodfellow, and my grandparents, Ken and Rona Eiliv Larsson (NGU) for approving my collaboration Bowie, who only ever gave me love and support. with Ola Fredin and Marc-Henri Derron, and Neil You will always be missed. Relict surfaces and blockfields / northern Swedish mountains 27
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Felsenmeer In: Ives, J.D. and Barry, R.G. (Eds.), Arctic and Alpine persistence under non-erosive ice in the Torngat and Environments. Methuen, London, pp. 605-636. Kaumajet mountains, Quebec and Labrador, as determined Jansson, K. and Glasser, N.F., in press. Modification of peripheral by soil weathering and cosmogenic nuclide exposure dating. mountain ranges by former ice sheets: The Brecon Beacons, Canadian Journal of Earth Sciences 41, 19-38. Southern UK. Geomorphology. Matsuoka, N., 1990. The rate of bedrock weathering by frost Jansson, K.N., Dahlgren, K.I.T., Stroeven, A.P., Alm, G., action: field measurements and a predictive model. Earth Glasser, N.F. and Goodfellow, B.W., in review. Using GIS Surface Processes and Landforms 15, 73-90. filter modelling for portraying the distribution of erosion Matsuoka, N., 2001. Direct observation of frost wedging in Relict surfaces and blockfields / northern Swedish mountains 29
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Basic Research cosmogenic radionuclide evidence of tors preserved through Opportunities in Earth Science. National Academy Press, multiple glacial cycles. Geomorphology 44, 145-154. Washington. Stroeven, A.P., Fabel, D., Harbor, J., Hättestrand, C. and Ollier, C.D. and Pain, C.F., 1997. Equating the basal Kleman, J., 2002b. Reconstructing the erosion history unconformity with the palaeoplain: a model for passive of glaciated passive margins: applications of in situ margins. Geomorphology 19, 1-15. produced cosmogenic nuclide techniques. In: Doré, A.G., Olvmo, M., Lidmar-Bergström, K., Ericson, K. and Bonow, Cartwright, J.A., Stoker, M.S., Turner, J.P. and White, N. J.M., 2005. Saprolite remnants as indicators of pre-glacial (Eds.), Exhumation of the North Atlantic Margin: Timing, landform genesis in southeast Sweden. Geografiska Annaler Mechanisms and Implications for Petroleum Exploration, 87A, 447-460. Special Publications 196. Geological Society, London, pp. Paasche, Ø., Strømsøe, J.R., Dahl, S.O. and Linge, H., 2006. 153-168. Weathering characteristics of arctic islands in northern Sugden, D.E., 1974. Landscapes of glacial erosion in Greenland Norway. Geomorphology 82, 430-452. and their relationship to ice, topographic and bedrock Phillips, W.M., Hall, A.M., Mottram, R., Fifield, L.K. and conditions. In: Brown, E.H. and Waters, R.S. (Eds.), Sugden, D., 2006. Cosmogenic 10Be and 26Al exposure Progress in Geomorphology: Papers in Honour of David ages of tors and erratics, Cairngorm Mountains, Scotland: L. Linton Special Publication No. 7. Institute of British Timescales for the development of a classic landscape of Geographers, London, pp. 177-195. selective linear glacial erosion. Geomorphology 73, 224- Sugden, D.E., 1978. Glacial erosion by the Laurentide Ice Sheet. 245. Journal of Glaciology 20, 367-391. Rea, B.R., Whalley, W.B. and Porter, E.M., 1996a. Rock Sugden, D.E., 1989. Modification of old land surfaces by ice weathering and the formation of summit blockfield slopes sheets. Zeitschrift für Geomorphologie Supplementband in Norway: Examples and implications. In: Anderson, M.G. 72, 163-172. and Brooks, S.M. (Eds.), Advances in Hillslope Processes Sugden, D.E. and Watts, S.H., 1977. Tors, felsenmeer, and vol 2. John Wiley and Sons Ltd, Chichester, pp. 1257- glaciation in northern Cumberland Peninsula, Baffin Island. 1275. Canadian Journal of Earth Sciences 14, 2817-2823. Rea, B.R., Whalley, B., Rainey, M.M. and Gordon, J.E., 1996b. Sugden, D.E., Balco, G., Cowdery, S.G., Stone, J.O. and Sass III, Blockfields, old or new? Evidence and implications from L.C., 2005. Selective linear erosion and weathering zones some plateaus in northern Norway. Geomorphology 15, in the coastal mountains of Marie Byrd Land, Antarctica. 109-121. Geomorphology 67, 317-334. Roaldset, E., Pettersen, E., Longva, O. and Mangerud, J., 1982. 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Periglacial features on the margins of a receding plateau ice cap, Lyngen, North Norway. Journal of Glaciology 27, 492-496. White, S.E., 1976. Is frost action really only hydration shattering? Arctic and Alpine Research 8, 1-6. Relict surfaces and blockfields / northern Swedish mountains 31
Appendix
The following is a summary of research conducted as part of my PhD study, but found to be non-central to the theme of the final thesis. Manuscript in preparation forEarth Surface Processes and Landforms.
Vertically mixed and unmixed: Do surface features tell the whole story? An investigation of till depth profiles using in situ-produced cosmogenic radionuclides
Bradley W. Goodfellow1, Derek Fabel2, Arjen P. Stroeven1, Richard Bintanja3, Marc W. Caffee4
1Department of Physical Geography and Quaternary Geology, Stockholm University, Sweden. 2Department of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK. 3Royal Netherlands Meteorological Institute, The Netherlands. 4Department of Physics, Purdue University, West Lafayette, Indiana, USA.
Introduction impossible to calculate the erosion rate or age of a partially mixed regolith. Surface features, such as The degree of vertical mixing is crucial to periglacially-sorted circles, may indicate full regolith determining a regolith erosion rate or age from in mixing, whereas an unsorted summit blockfield may situ-produced cosmogenic radionuclides. The surface represent an unmixed regolith. nuclide concentration is the same for a fully mixed in In this study, we examined the mixing of a situ regolith as it is for an unmixed regolith (Granger periglacially sorted till blanketing the summit of et al., 1996), and a regolith erosion rate or age can Ruohtahakčorru (68°09´N, 19°20´E; 1349 m a.s.l.) be calculated. However, without further information in the northern Swedish mountains (Fig. 1). We on the mixing history of each regolith sample, it is used 10Be and 26Al in quartzite clasts extracted from
0IT 0IT
0IT
10ºE 20ºE 30ºE 0IT 70ºN 0IT
Study site
65ºN
Sweden Finland Norway 60ºN
55ºN
Fig. 1. Location of the study site in the northern Swedish mountains and pit excavations 1 and 2 into till on the plateau surface of Ruohtahakčorru. 32 B.W. Goodfellow / Doctoral Dissertation / 2008
depth profiles to determine mixing degree and to
. . calculate an approximate period of till emplacement. -2 Be Be 10 We anticipated full mixing of the unconsolidated till 10
Al/ attributable to clast upfreezing and/or lateral sorting 26 5.5 ± 0.5 4.7 ± 0.4 4.2 ± 0.4 6.0 ± 0.8 5.8 ± 0.5 6.1 ± 0.5 3.3 ± 0.3 5.3 ± 0.5 6.2 ± 0.5 of clasts into sorted circles.
Pit excavation and regolith sampling year) 3 9.7 ± 0.9 8.4 ± 0.8 (10 29.9 ± 2.9 17.4 ± 1.6 24.2 ± 2.2 24.6 ± 2.2 17.9 ± 1.7 10.9 ± 1.0 Al apparent 21.6 ± 0.43 26 exposure age Two pits were hand excavated; one to a depth of 1.30 m in the clast-dominated ring of a sorted circle and the second to a depth of 1.10 m in a fine matrix- Be concentrations were normalised were concentrations Be year) 10
3 dominated sorted circle centre (Fig. 1). In each pit, 9.4 ± 1.1 (10 36.5 ± 2.9 25.1 ± 1.8 15.9 ± 1.3 28.1 ± 2.2 27.2 ± 2.1 37.1 ± 2.9 13.9 ± 1.1 23.4 ± 1.8 Be apparent excavation depth was limited by the presence of exposure age 10 large rock slabs. Four quartzite clasts were taken
-1 from various depths in Pit 1 and five from various ) and an attenuation mean free path of 150 g cm 5 10 26 -3 depths in Pit 2 for measurement of Be and Al
(x 10 concentrations. Al] atom g 9.16 ± 0.67 5.35 ± 0.37 3.01 ± 0.21 2.59 ± 0.19 7.44 ± 0.48 7.55 ± 0.48 5.51 ± 0.38 3.38 ± 0.23 6.63 ± 0.43 26 [
-1 Unmixed and fully mixed regolith ) 5 Both 10Be and 26Al data indicate almost completely (x 10 Be] atom g 1.66 ± 0.09 1.15 ± 0.05 0.71 ± 0.04 0.43 ± 0.04 1.28 ± 0.06 1.24 ± 0.06 1.69 ± 0.08 0.64 ± 0.03 1.07 ± 0.05
10 unmixed regolith in Pit 1 (Table 1; Fig. 2). Conversely, [ three samples from Pit 2 are consistent with a fully mixed regolith, while one sample lies almost on the unmixed curve. The latter was taken from a large (> factor 3.489 3.489 3.489 3.489 3.480 3.480 3.480 3.480 3.480 Al scaling
26 2 m) boulder. A fifth sample was excluded because of a large discrepancy between measured 10Be and 26Al concentrations, which may indicate nuclide factor 3.495 3.495 3.495 3.495 3.487 3.487 3.487 3.487 3.487 inheritance from a period prior to incorporation into Be scaling
10 the till. Caution must be exercised in assuming that , respectively (Stone, 2000; Nishiizumi et al., 2007). All measured All 2007). al., et Nishiizumi 2000; (Stone, respectively , -1 an entire regolith is either unmixed or mixed from yr -1 interspersed depth profiles and the degree of mixing 0.966 0.966 0.966 0.966 0.966 0.966 0.966 0.966 0.966 correction
Thickness may differ significantly from that indicated by surface features. (m) 0.50 1.05 1.23 0.20 0.50 0.55 1.05
Depth 26 Surface Surface Fitting Al data to an unmixed curve
Our model of the Pit 1 unmixed profile, which is based on a regolith density of 2.22 g cm-3 and an Lithology Quartzite Quartzite Quartzite Quartzite Quartzite Quartzite Quartzite Quartzite Quartzite attenuation mean free path of 150 g cm-2, indicates a close, but inexact, match with the measured 26Al
1346 1346 1346 1346 1343 1343 1343 1343 1343 data (Fig. 3). Accounting for muon production (m a.s.l.) Elevation partly closes the gap between the measured data and the unmixed curve. However, a near-perfect fit is attained at a realistic regolith density of 2.10 g no.
Sample -3
BG-04-01 BG-04-02 BG-04-03 BG-04-04 BG-04-05 BG-04-06 BG-04-07 BG-04-08 BG-04-09 cm but with an excessively high attenuation mean
Al production rates of 4.6 ± 0.3 and 31.1 ± 1.9 atoms g atoms 1.9 ± 31.1 and 0.3 ± 4.6 of rates production Al -2
26 free path of 190 g cm . Other possible scenarios, 1 1 1 1 2 2 2 2 2 Pit no. Table 1: Cosmogenic nuclide data, apparent exposure ages, and ratios from till sections, Ruohtahakčorru, northern Sweden. Table Topographic shielding corrections < Thickness 0.1%. corrections were calculated using a rock density of Topographic 2.75 g cm high-latitude sea-level to normalised were Data °C. 5 of temperature surface sea a with (2000) Stone to according calculated were factors scaling Altitude-latitude and SRM 4325. to NIST including starting the unmixed curve from the upper error limit of the surface sample, do not produce such Relict surfaces and blockfields / northern Swedish mountains 33
0 0 Unmixed regolith Unmixed regolith -0.2 Mixed regolith -0.2 Mixed regolith Sample Sample -0.4 -0.4 -0.6 -0.6 -0.8 Regolith depth (m) Regolith depth (m) -0.8 -1.0
-1.2 -1.0
0 0.5 1.0 1.5 2 0 0.5 1.0 1.5 5 5 10Be N (at g-1) x 10 10Be N (at g-1) x 10
0 0 Unmixed regolith Unmixed regolith -0.2 Mixed regolith Mixed regolith -0.2 Sample Sample -0.4 -0.4 -0.6 -0.6 -0.8 Regolith depth (m) Regolith depth (m) -0.8 -1.0
-1.2 -1.0
0 2 4 6 8 10 0 2 4 6 8 5 5 26Al N (at g-1) x 10 26Al N (at g-1) x 10
Fig. 2. 10Be (top) and 26Al (bottom) concentrations with regolith depth for Pit 1 (left) and Pit 2 (right), Ruohtahakčorru, northern Sweden. Vertical dashed lines represent fully mixed profiles based on surface nuclide concentrations and adjusted within surface error margins to fit the measured data. Solid lines represent unmixed profiles with surface nuclide concentrations equal to those for the fully mixed profiles. Constant regolith density (ρ = 2.22 g cm-3) is assumed and an attenuation mean free path of 150 g cm-2 is used. Error bars show 1σ uncertainty and all data have been normalised to sea-level high latitude production rates of 4.6 ± 0.3 and 31.1 ± 1.9 atoms g-1 yr-1 for 10Be and 26Al, respectively (Stone, 2000; Nishiizumi et al., 2007). a close fit. Matching a realistic modelled curve with inherited from the bedrock below as bedrock the 26Al data, therefore, remains out of reach. fragments become incorporated into the regolith
(Nbr) and the depth-averaged nuclide production rate 10 Indications of regolith origins from Be and in the regolith (Preg) multiplied by the mean regolith 26 Al residence time (treg), i.e.:
Combined pit data indicate that the regolith has Nreg = Nbr + Pregtreg (Granger et al., 1996). not been produced in situ and is a till (Fig. 4). In an in situ regolith, the cosmogenic nuclide concentration Because the fully mixed profile has lower through the fully mixed profile is the same as the nuclide concentrations than the unmixed profile, surface nuclide concentration of an unmixed profile. the regolith appears not to have been produced This is because the regolith nuclide concentration in situ. Furthermore, the difference between the in a fully mixed profile (Nreg) is the sum of nuclides nuclide concentration in the fully mixed profile and 34 B.W. Goodfellow / Doctoral Dissertation / 2008
0 0 Unmixed pro le -0.2 -0.2 Fully xed pro le Fully mixed pit 2 -0.4 -0.4 Pit 1 Pit 2 -0.6 -0.6 Unmixed profile ρ = 2.22 g cm-3 Attenuation mean -0.8 free path = 150 g cm-2 -0.8 Muon production Regolith depth (m) -1.0 Muon production Regolith depth (m) -1.0 ρ = 2.10 g cm-3 Attenuation mean Nbr Nbr -2 free path = 190 g cm -1.2 -1.2 Sample 1 2 3 4 5 6 7 8 9 10 0 0.5 1 1.5 2 5 5 26Al N (at g-1) x 10 10Be N (at g-1) x 10
26 0 Fig. 3. Normalised Pit 1 Al concentrations with regolith depth. Unmixed pro le The first two curves (from the left) show an unmixed profile Fully xed pro le and an unmixed profile that also accounts for muon production. -0.2 Fully mixed pit 2 Both of these profiles are based on a regolith density of 2.22 g Pit 1 cm-3 and an attenuation mean free path of 150 g cm-2. The third -0.4 curve illustrates an unmixed profile that matches the measured Pit 2 data. This curve accounts for muon production and is based on -0.6 a realistic regolith density of 2.10 g cm-3 but an excessively high -2 attenuation mean free path of 190 g cm . -0.8 the surface nuclide concentration of the unmixed Regolith depth (m) -1.0 N N profile equals, within error margins, the nuclide br br -1.2 concentration at 1.10 m depth on the unmixed curve, which is equivalent to the fully mixed regolith base. 0 2 4 6 8 10 5 The difference in nuclide concentrations between 26Al N (at g-1) x 10 the profiles therefore equals the missing addition of nuclides from the underlying bedrock. The regolith Fig. 4. Combined Pit 1 and Pit 2 10Be concentration profiles 26 in each pit may have a common origin and deposition (top) and Al concentration profiles (bottom). Vertical dashed lines represent fully mixed profiles based on surface nuclide during a single glaciation is indicated. concentrations and adjusted within surface error margins to best fit the measured data. Solid lines represent unmixed profiles When was the till emplaced? originating from the low error limit of the Pit 1 surface sample to best fit measured data. Constant regolith density (ρ = 2.22 g -3 The 10Be concentration of the Pit 1 (unmixed cm ) is assumed. Error bars show 1σ uncertainty and all data have been normalised to sea-level high latitude production rates profile) surface sample can be used to estimate when of 4.6 ± 0.3 and 31.1 ± 1.9 atoms g-1 yr-1, respectively (Stone, the till was emplaced if the surface erosion rate, 2000; Nishiizumi et al., 2007). The distance Nbr illustrates durations of surface burial by ice sheets, and glacial that the nuclide concentration at the base of the unmixed Pit 1 isostasy can also be estimated. An erosion rate of profile, which is equivalent to the concentration of nuclides that 8–10 mm kyr-1 is estimated from a nearby summit would be contributed by bedrock as it is incorporated into an -1 in situ-produced regolith, is the same as the difference, within blockfield erosion rate of 4 mm kyr (Paper IV), error margins, between fully mixed profiles for Pits 1 and 2. It which has been adjusted upwards to account for the is therefore possible that the sampled till from both pits was greater abundance of fine matrix. Erosion rates of deposited during the same glaciation. similar magnitude have been calculated on another, fine matrix-rich, periglaciated plateau surface blanketing Ruohtahakčorru, whereas a faster erosion (Small et al., 1999). A slower estimated erosion rate rate will increase the residence time of the till. Burial will reduce the time that the sampled till has been durations and glacial isostasy have been estimated Relict surfaces and blockfields / northern Swedish mountains 35
2000
1750
1500
1250
1000
750
500
250 Elevation (m above present sea level) 0 0 1 2 3 4 5 6 7 8 9 10 11 5 Ice surface elevation Time (years before present) x 10 Ice thickness (m) Bed elevation
Fig. 5. Modelled ice sheet surface elevation, ice thickness, and bedrock response to ice sheet loading and unloading for Ruohtahakčorru. The ice sheet model is forced by benthic δ18O records, has a 40 km resolution, and simulates bedrock response through an elastic lithosphere, relaxed asthenosphere (ELRA) model (Bintanja et al., 2002, 2005). using a 3-dimensional numerical ice-sheet model Stone, J.O., 2000. Air pressure and cosmogenic isotope forced by benthic δ18O records and incorporating an production. Journal of Geophysical Research 105, 23753- elastic lithosphere, relaxed asthenosphere (ELRA) 23759. model to simulate bedrock movement (Fig. 5; Bintanja et al., 2002, 2005). From the 10Be concentration of the Pit 1 surface sample, and accounting for erosion, burial, and glacial isostasy as indicated, till emplacement is attributed to the Saalian glaciation, which occurred between 300 and 130 kyr ago. It has survived subsequent glaciation through being covered by cold-based ice.
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