Do periglacial landscapes evolve under periglacial conditions?

Geomorphology 52 (2003) 149 – 164 www.elsevier.com/locate/geomorph

Do periglacial landscapes evolve under periglacial conditions? Marie-Francßoise Andre´ * Laboratory of Physical Geography, UMR 6042-CNRS, Blaise Pascal University, 4 rue Ledru, F-63057 Clermont-Ferrand Cedex 1, France Received 6 February 2002; received in revised form 12 May 2002; accepted 14 August 2002

Abstract Polar and alpine periglacial areas are traditionally regarded as necessarily submitted to very efficient frost-driven processes that control the Holocene and ongoing geomorphic activity. During the last decade, the validity of this academic view has been increasingly questioned among the international scientific community. The search for the real past and present processes responsible for landform evolution in cold nonglaciated areas is based mainly on more and more refined monitoring protocols relying upon sophisticated equipment. To assess the representativeness and significance of monitoring data collected in restricted sites, it appears necessary, however, to widen the perspective by adopting a twofold multiscale approach as proposed in the present paper: (1) in space, by integrating various scales from the general slope system to the bioclimatic nanoenvironment; and (2) in time, by taking into account the landscape history, from Tertiary inherited features to recent process changes induced by the contemporary warming. Reintegrating the historical approach should help both to place the ongoing processes within a succession and/or combination of interoperating processes, and to avoid misinterpretations of features considered wrongly as emblematic of frost action. Overall, both the historical and monitoring approaches tend to reduce the geomorphic efficiency of frost-derived processes. Instead, the role of noncold-related processes, such as chemical, thermal, biogenic and rainfall-induced ones, is being emphasized. Of special interest would be an interdisciplinary discussion between geomorphologists and various disciplines of biological sciences including biochemistry and ecology studying processes of primary successions. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Landscapes; Periglacial; Holocene

1. Historical background and need for a revision of the ‘‘freeze– thaw dogma’’ Due to their cold-climate conditions and their fascinating seasonally derived and permafrost-derived *

Fax: +33-473-346-824. E-mail address: [email protected] (M.-F. Andre´).

patterned ground, periglacial areas are usually regarded as parts of the world where landscape evolution is controlled primarily by efficient freeze – thaw mechanisms. Frost shattering and related processes such as cryoplanation and nivation are systematically emphasized. Their responsibility in the formation of a wide range of landforms and deposits such as Richter slopes, cryopediments, blockfields and rockglaciers is also taken for granted. Conversely, more

0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-555X(02)00255-6

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ubiquitous noncold-related processes like biochemical weathering and rainfall-induced slope processes are systematically underestimated. This trend is a long-lasting one as shown by Etienne (2001a,b; cf. Fig. 1) who compiled textbooks dealing with cold nonglacial environments from Tricart (1963) to French (1996).

However, even during the ‘‘periglacial fever’’ of the 1960s, scientists happened to criticize the overrating of the role of freeze –thaw mechanisms in the interpretation of cold-region geomorphic features. Based on process monitoring in the famous valley of Ka¨rkevagge (northern Sweden), Rapp (1960a) demonstrated that solution was much more important

Fig. 1. The importance of frost-derived processes in the erosion system of cold nonglacial environments, based on the contents of ‘‘periglacial’’ textbooks 1963 – 1996 (Etienne, 2001b, p. 28; Embleton and King, 1975; French, 1976; Hamelin and Cook, 1967; Tricart, 1981; Tricart and Cailleux, 1967; Washburn, 1973; Washburn, 1979).

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than mechanical processes. Czeppe (1964) stated that frost mechanisms failed to account for rock flaking on Spitsbergen, as did Malaurie (1968) in Greenland, who sought in Sahara for analogues of this weathering phenomenon. More recently, three papers of general interest were published whose titles illustrate the growing concern of the international community in this matter:

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background to the bioclimatic nanoenvironment; and (2) in time, by taking into account the landscape history, from inherited landforms and deposits dating back to the Tertiary warm climates to changes in combinations of geomorphic processes deriving from Late Holocene climate fluctuations.

2. Spatial heterogeneity and process diversity 

Nivation: a geomorphic chimera (Thorn, 1988),  Periglacial geomorphology: what, where, when? (Thorn, 1992),  Freeze –thaw weathering: the cold region ‘‘panacea’’ (Hall, 1995). The ability to generate hollows and terraces cut in bedrock with processes emblematic of cold environments such as nivation and cryoplanation is more and more questioned. ‘‘Modern use of the term nivation is not recommended’’, French (1996, p. 159) even states in the last edition of his ‘‘Periglacial Environment’’. Both the reality of frost-related mechanisms and their position within the hierarchy of processes interoperating in periglacial environments definitely need clarification and revision. Two ways should help to cross ‘‘the smokescreen of the periglacial scenery’’ (Andre´, 1999) to search for the real past and present processes responsible for the landforms and deposits represented in the so-called ‘‘periglacial’’ areas. The first one is based mainly on improvements of the monitoring protocols. New methods make it possible to investigate all the processes potentially involved in rock weathering, including the biogenic ones and those, thermally driven, known to occur in hot desert environments (Hall, 1999). The second way to break with the stereotypical view of the frost-driven landscape evolution is to set the ongoing space-restricted monitoring studies within a wider framework, both in space and in time. Bridging the gap between process and historical geomorphology might help to revise previous interpretations of cold-region geomorphic features as well as to assess the representativeness and significance of current monitoring data. Some guidelines are proposed in the present paper to widen the perspective by adopting a twofold multiscale approach: (1) in space, by integrating various scales from the general slope system and geological

One of the reasons for the reductionist model of the so-called ‘‘periglacial erosion system’’ probably lies in the conditions involved in the development of periglacial geomorphology. Many pioneer studies were carried out in specific contexts, most suitable to frost expression, which do not reflect the great variety of geological backgrounds and environmental conditions to be found in cold areas. Moreover, climatic conditions were often examined at a regional scale, based on unsuitable air temperature data and insufficient moisture data, as stressed by various authors (e.g., Thorn and Hall, 1980; McGreevy and Whalley, 1985; Hall, 1986; Matsuoka, 1990). 2.1. Slope system diversity and rock control of geomorphic processes Densely jointed and/or porous limestone and chalk of European sedimentary basins were widely investigated during the golden age of the periglacial geomorphology which accompanied and followed the birth of climatic geomorphology. In France for instance, Tricart’s doctoral thesis on the eastern Parisian Basin and Guillien’s Quaternary studies of the Charente gre`zes lite´es led the whole scientific community to emphasize frost action whose efficiency on carbonates was further experimentally demonstrated at the Centre de Ge´omorphologie de Caen (Tricart, 1949/ 1952; Guillien, 1964; Lautridou and Ozouf, 1982). However, laboratory experiments performed in Caen demonstrated that the behaviour of carbonates was not applicable to low porosity crystalline rocks, like granite and gneiss, which are most widespread in nordic and arctic shields. At Caen, 50 rock samples from Labrador and Spitsbergen underwent 1120 freeze – thaw cycles at 5 jC, with resulting debris amounts ranging over three orders of magnitude. On average, sedimentary rocks produced 35 times more

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debris than crystalline rocks of which many remained intact until the end of the experiment (Andre´, 1993; cf. Fig. 2). The same contrast was observed in the field among erratics of various lithologies which had all experienced Holocene weathering in the Torngat Mts (Labrador). While dolomite erratics had been completely destroyed or split into slices, the morphology of gneiss boulders had been preserved under their lichen cover which only generated a pellicular biogenic granular disintegration (Andre´, 1982). Beyond the rock control associated with porosity and/or jointing, the major contrast between the slope systems of polar Precambrian shields and alpine mountain ranges has to be considered when investigating weathering processes and assessing their spatial representativeness. The first ones have a main horizontal component, for they derive from planation surfaces gently scoured by ice sheets into subdued ‘‘knob and basin’’ topographies. In this context, with exception for some wet places having experienced frost heave (e.g.,

Fig. 2. Contrasted debris production from various lithologies after 1120 freeze – thaw cycles at 5 jC (Andre´, 1993, p. 144). Experiments were carried out at the Centre de Ge´omorphologie de Caen.

Dionne, 1981; Michaud and Dionne, 1987), massive and nonporous crystalline ice-scoured outcrops do not show any particular susceptibility to frost-derived mechanisms. In northern Scandinavia, during the last 10 000 years, such outcrops have undergone only a very slow biogenic weathering, resulting in an overall millimetric to centimetric surface lowering (Table 1). Weathering rates of 0.0002 to 0.001 mm a 1 were found in granite of the Narvik Mountains and surroundings, based on the use of ice-polished quartz veins as reference surfaces (Dahl, 1967; Andre´, 1995a). In quartzite, postglacial weathering has been so negligible that even the finest glacial striations are preserved on top of the Late Weichselian roches moutonne´es. The ubiquity of biogenic weathering might explain the similarity of postglacial rates of bedrock lowering in arctic and temperate areas. In the Late Wisconsinian roches moutonne´es of the Sherbrooke area (southern Quebec), Cle´ment et al. (1976) found weathering rates ranging from 0.0005 to 0.0012 mm a 1 for various metamorphics, with the lowest rates in quartzitic sandstone. In the Go¨teborg area (S Sweden), weathering rates reported by Lindberg and Brundin (1969) and Rudberg (1970) for micaschist and gneiss reach 0.0015 mm a 1, i.e. the same as biotite-rich metamorphics of northern Scandinavia. Conversely, slope systems of the high-alpine areas are dominated by a vertical component that accounts for the importance of mechanical processes, although the role of jointing inherited from tectonic stress, frost action, pressure release and other potentially active processes is difficult to assess. Whatever the underlying reason, most of the alpine mountain walls have provided abundant debris since the last deglaciation, and fed scree slopes and rockglaciers, resulting in rates of retreat averaging 1 mm a 1, i.e. 1000 to 50 000 times more rapid than the rates of surface lowering having affected the subdued arctic crystalline outcrops (Table 2). However, such rapid retreat rates are not totally absent from the Arctic where they are found in the alpine-like accented uplifted margins of the nordic shields where postglacial pressure release mechanisms have affected the fjord walls (see below). 2.2. From mesoclimates to nanoenvironments On a macroregional scale, polar and alpine environments are usually described as dominated by

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Table 1 Rates of biogeochemical bedrock lowering in nordic and arctic areas Location

Metamorphics N Sweden, 68jN N Norway, 68jN N Sweden, 68jN N Sweden, 68jN N Norway, 68jN N Norway, 68jN Sedimentary rocks W Spitsbergen, 78jN N Sweden, 68jN Hudson Bay, 56jN

Lithology

Rates of Holocene bedrock lowering (mm ka 1)

Source

Min – Max

Mean

quartzite acid granite amphibolite phyllite granite biotite-rich granite

0.1 – 0.6 0.1 – 0.5 0.1 – 1.0 0.2 – 2.0 ? 0.4 – 3.7

0.2 0.2 0.3 0.8 1.0 1.1

Andre´, 1996 Andre´, 1995a Andre´, 1996 Andre´, 1996 Dahl, 1967 Andre´, 1995a

dolomitic limestone dolomite dolomite

1.5 – 3.8 1.7 – 13.0 3.1 – 11.0

2.5 5.3 6.0

˚ kerman, 1983 A Andre´, 1996 Dionne and Michaud, 1986

Processes responsible for bedrock lowering are granular disintegration in metamorphics and solution in carbonated sedimentary rocks. Icepolished quartz veins were used as reference surfaces dating back to 10 000 years except for Quebec (6500 years) according to radiocarbon dating.

mechanical processes of rock breakdown, in contrast with the humid tropics, a realm of chemical weathering. Such a schematic view was promoted by Peltier (1950) and Strakhov (1967), whose classical sketches were interestingly criticized by Pope et al. (1995).

Strakhov’s sketch (Fig. 3a) assigns to regional trends in vegetation and climate the contrast existing between the thin polar soils and the deep equatorial profiles. Such a view underestimates the role of time: in contrast with inherited tropical regoliths, arctic

Table 2 Rates of mechanical rockwall retreat in alpine areas Location

Lithology

Temperate mountains Swiss Alps Swiss Alps Swiss Alps Austrian Alps Austrian Alps French Alps French Alps French Alps

granite, schist granite, schist various gneiss, schist various gneiss granite schist

Arctic mountains Central Spitsbergen SW Ellesmere Mts W Greenland Mts W Greenland Mts

limestone limestone volcanics basalt

Rates of Holocene rockwall retreat (mm ka 1)

Source

Min – Max

Mean

1000 – ? 1500 – 3400 800 – 1500 700 – 1000 ? – 5000 – – –

2500 – – – – 1000 2500 1200

Barsch, 1977 Barsch, 1996 Barsch, 1996 Poser, 1954 Buchenauer, 1990 Francou, 1988 Francou, 1988 Kaiser, 1992

340 – 500 500 – 1300 500 – 1500 ? – 6000

– – – 2000

Rapp, 1960b Souchez, 1971 Frich and Brandt, 1985 Humlum, 2000

Evaluations are based on the volume of talus cones and rock glaciers developed since deglaciation, i.e. between 3000 and 15 000 BP according to the locations. Frost shattering, stress relaxation mechanisms and noncold processes exploiting the preexisting joints are involved in the rockwall retreat.

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importance of thermal stress in cold and dry environments (Hall, 1999). On the whole, it appears necessary to work at various scales and to combine climatic and geological data as illustrated by the example of the Dry Valleys of continental Antarctica. The regional aridity of these oases swept by the catabatic winds explains the importance of thermal weathering and wind erosion. Locally, the occurrence of alveolar weathering (taffonis) can be explained by the distribution of salts related to the geological and topographical pattern. Last, at a nanoscale, life can be responsible for rock flaking where cryptoendoliths meet suitable conditions for colonization. Moreover, combining the geological and environmental conditions helps us to understand the distribution of weathering processes that are more efficient, where the substrate is the most favourable: dark rocks such as dolerite for thermal weathering, grained rocks such as granite for salt weathering, and translucent and porous rocks like the Beacon sandstones for bioweathering. Fig. 3. Strakhov (1967)’s and Peltier (1950)’s diagrams: a theoretical and reductionist view of the world-wide distribution of weathering processes (in Pope et al., 1995, p. 39).

soils are thin mainly because they are young, due to their periodic rejuvenation during glacial episodes. As to Peltier’s diagram (Fig. 3b), it presents a world distribution of weathering processes related to atmospheric indicators (air temperature and annual precipitation) which are very different from the real climate conditions experienced by rock surfaces. Site conditions and nanoenvironments are much more significant, especially in ‘‘extreme’’ environments like cold and hot deserts where rock temperature and water supply vary greatly from one place to the other. Since Geiger’s (1965) pioneer book, The Climate Near The Ground, microclimate monitoring has tremendously improved, for example in Antarctica where ‘‘nanoclimate’’ data—including rock temperature, wind and moisture—were first collected by the biologists investigating the cryptoendolithic microbial ecosystems (e.g., Friedmann et al., 1987). Subsequently, the geomorphologists monitored rock climate conditions with increasing resolution (e.g., Hall, 1997; Hall and Andre´, 2001), which led them to demonstrate the

3. Towards bridging the gap between process and historical geomorphology The contemporaneous shift of geomorphology from a descriptive and historical approach to modern studies based on process monitoring enhanced uniformitarian perspectives. Many scientists did not take into account any more the persistence of landforms throughout the geological times, and inferred wrongly present-day frost mechanisms from inherited geomorphic features such as blockfields. Conversely, the genesis of landforms like the so-called ‘‘cryopediments’’ was deduced from observations made on recent superficial deposits covering surfaces with which no causal link had been established, as shown by French and Harry (1992) in Yukon. There is an urgent need to bridge the gap between historical and process geomorphology to avoid confusions and misinterpretations (Rhoads and Thorn, 1996). Process geomorphology dealing with weathering and slope dynamics should reintegrate the history of landscapes by combining monitoring studies with geomorphological mapping. According to the study sites, this historical perspective should be

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applied at various time scales, from the minor Late Holocene glacioclimatic fluctuations to the pre-Quaternary inheritances. 3.1. A short-term perspective: the case of the contemporaneous warming and of the resulting paraglacial ‘‘crisis’’ Following the Little Ice Age, the 20th century has known two thermal peaks in the late 1930s and the 1990s, well marked in the Scandinavian Arctic (Lapland and Spitsbergen). In this part of the world, the contemporaneous warming was accompanied by a northward shift of the polar front, which induced an increase of rainstorm episodes and associated debris flows and slides (Rapp and Nyberg, 1988; Andre´, 1995b). As early as the 1960s, some geomorphologists working on Spitsbergen suggested that a succession of processes might have accompanied this climate change. Thus, in the Kongsfjord area, Gabert and Masseport (1966) suggested that landforms and deposits created by frost action were being reworked or destroyed by runoff processes. Raynal (1969) even used the expression of mutation de processus, pointing to the possibility of a radical change within the erosion system. The idea of a contemporaneous change in the hierarchy of geomorphic processes operating on Spitsbergen slopes was further developed by Andre´ (1993) based on four main indicators: 1. The 100-year-old lichens (Rhizocarpon geographicum) that colonize densely jointed rockwall elements whose retreat, possibly due to frost shattering, has stopped since the end of the Little Ice Age; 2. The centimetre-wide solution pits on limestone clasts located in the upper part of scree deposits whose accretion has obviously stopped; 3. The gentle reworking by snowmelt waters of the surface of scree slopes which are not fed anymore by the mountain walls that tend to be densely colonized by lichens, algae and cyanobacteria; 4. And more importantly, the dissection and fossilization of rockglaciers and scree cones by debris flow channels and lobes generated by summer heavy rainfalls (Fig. 4).

Fig. 4. Fossilization (right) and dissection (middle) of talus-foot rock glaciers by debris flow cones and channels on the northwestern coast of Spitsbergen (Kapp Mitra, 79jN).

While chemical and biological agents seem to be the main present-day contributors to weathering, runoff sensu lato is by far predominant among the ongoing slope processes on Spitsbergen where freeze – thaw mechanisms do not currently seem to play any significant role in debris production or transit. In recently deglaciated areas of Spitsbergen and continental Norway, paraglacial runoff generates high erosion rates due to the abundance of loose debris and meltwater (Ballantyne and Benn, 1994; Curry, 1999; Mercier, 2001). Although spectacular, gullies, leve´es and lobes created in paraglacial conditions (Fig. 5) are shortlived landforms whose activity stops as soon as the water source is ‘‘extinct’’. In northwestern Spitsbergen, the activity of gullies created by meltwater from ice-cored lateral moraines was shown to last for no more than 20 years (Mercier, 1997). The contemporaneous debris flow activity, which is a major component of the ongoing erosion system of arctic areas, is quite similar to that occurring in the

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Fig. 5. Gully and associated leve´es formed in morainic material by dead-ice melting during the contemporary deglaciation phase (Kongsfjord, NW Spitsbergen, 79jN).

French Alps, the Polish Tatras or the Uluguru Mountains in Tanzania (e.g., Van Steijn et al., 1988; Kotarba, 1991; Temple and Rapp, 1972). These are ubiquitous rainfall-triggered phenomena, which are not related to the cold-climate conditions. However, in polar areas, the superficial permafrost-table permits small amount of precipitation to trigger debris flows; e.g., only 31 mm in 12 h in Longyeardalen (central Spitsbergen) on July 10 – 11, 1972 (Thiedig and Leh-

mann, 1973), against 100 mm in 3 h on February 23, 1979, in Tanzania (Rapp et al., 1991). 3.2. The intermediate time scale of the major Quaternary climate changes Beyond the brief paraglacial ‘‘crises’’ like the contemporaneous one, the major deglaciation periods determine landform evolution because of their long-

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lasting geomorphic impact. In the Canadian and Scandinavian Arctic, between 12 000 and 6000 BP, the Wisconsinian/Weichselian glacial processes were replaced by paraglacial processes associated with the ice sheet decay. Then, accompanying the vegetational recolonization of deglaciated areas, bio-physicochemical processes played an increasing role in debris production and transfer during the Holocene. Along the fjords dissecting the margins of the Fennoscandian and Canadian shields (Scandes and Torngat Mts), the most efficient paraglacial geomorphic process is large-scale bedrock exfoliation. Sheets of granite up to 3 m in thickness are expanding like giant onion skins, whose orientation, parallel to the fjords, points to postglacial pressure release as the main triggering mechanism. This induces rapid rockwall retreat, up to 1 mm a 1 on Spitsbergen, based on debris volume and lichenometric chronological control (Andre´, 1997), i.e. on the same order of magnitude as the rockwall retreat rates of midlatitude alpine areas (see above). The rock types affected by exfoliation—mostly granite and gneiss—do not show any susceptibility to frost shattering owing to their massiveness (plurimetric jointing). Therefore, the widening of the valleys depends mainly on pressure release processes, which are independent of periglacial freeze – thaw mechanisms (even if they happen to interfere with them). Does pressure release operate only during deglaciation phases or also during glacial phases as suggested by Trainer (1973)? Can the 1mm-a 1 retreat rate found on Spitsbergen be applied to the major Scandinavian fjords? How long after the valley deglaciation are pressure release mechanisms still active? These are unsolved questions to date. Lichen colonization of many overhanging granite sheets dominating fjords suggests that pressure release is being replaced by biogenic weathering. However, some of the lichen-covered spalls happen to collapse, indicating that pressure release is still active, in some case, 10 000 years after the end of the glacial phase. Of special interest are sites of central and northwest Spitsbergen where scree deposits show a decrease in the clast size from the rockglacier fronts to the rockwalls supplying debris (diameters from 1.50 to 0.20 m on average). Lichenometric surveys suggest that the largest boulders have been delivered during rockglacier initiation, i.e. about 3500 –4000 BP, whereas the smallest, located right at the base of the mountain

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walls, were provided during the cold 1880 –1890s (end of the Little Ice Age) period. Since then, as mentioned hereabove, many rockwalls have been colonized by lichens, under which, they undergo a slow biogenic weathering. Based on both the chronological control and the debris pattern, it is tempting to consider a tripartite succession both in processes and associated rates of rockwall retreat (Andre´, 1997; cf. Table 3): 1. Postglacial pressure release responsible for the delivery of the plurimetric boulders of rockglaciers associated with an early neoglacial phase (1 mm a 1); 2. Frost shattering having produced decimetric clasts found in the upper part of rockglaciers and the overlying scree deposits dating back to the Little Ice Age (0.1 mm a 1); 3. Biogenic granular disintegration associated with the contemporaneous lichen colonization of stabilized rockwalls (0.001 mm a 1). Based on data from the Kongsfjord and Wijdefjord areas (79jN), this scenario is consistent with Rapp’s (1960b) previous estimates of rockwall retreat in the Tempelfjord area. He found a major discrepancy between the negligible contemporary rates Table 3 A triple-rate combination of weathering processes operating on Spitsbergen during the late Holocene (Andre´, 1997) Location

Lithology

Process

Wijdefjord, 79jN

amphibolite

biogenic flaking

Wijdefjord, 79jN Kongsfjord, 79jN

amphibolite

frost shattering? frost shattering?

Kongsjord, 79jN (Ossian Sarsfjellet nunatak)

quartzite

quartzite

postglacial stress relaxation

Rates of Holocene rockwall retreat (mm ka 1) Min – Max

Mean

0–4

2

30 – 110

70

110 – 220

160

100 – 1580

700

Evaluations are based on volumetric calculations of deposits, direct measurements of rockwall retreat and lichenometrical chronological control.

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based on photogrammetry (0.02 mm a 1 over the period 1882 – 1954) and the significant Holocene rates (0.5 mm a 1) inferred from the volume of the impressive scree cones and associated rockglaciers (Fig. 6). Although supported by numerous field data, the preceding scenario is, however, schematic and expresses no more than a trend, with one process being dominant within a combination of processes. For instance, pressure release and biogenic weathering have been shown to operate simultaneously on the rockwalls of the Norwegian fjords, but they do not deliver debris of the same size and with the same periodicity. Pressure release seems to have been more active in an early stage of the Holocene, whereas biogenic microweathering seems to have taken over more recently. Moreover, an embarrassing question remains unsolved, concerning the huge boulders deposited by pressure release that are found at the edge of the Spitsbergen rockglaciers. Why do all the lichenometric surveys point to a 3500– 4000-year-old delivery of these, while the deglaciation occurred 10 000 and even 40 000 years ago at places like Prins Karls Forland? How can we explain this delayed response of the mountain walls?

3.3. A long-term perspective: the persistence of Tertiary landforms and deposits Until recently, the age and origin of blockfields, felsenmeers (Fig. 7) and other mountain-top detritus have been heavily debated, particularly in Canada and Scandinavia (e.g., Sellier, 1995, 2002; Rea et al., 1996). Whereas Ives (1966) considered the Baffin and Labrador felsenmeers as old features associated with pre-Wisconsinian tors, Dahl (1966) suggested that the blockfields of the Narvik Mountains were the fresh products of Holocene frost shattering. This last interpretation is no longer valid for two main reasons: 1. Many Scandinavian blockfields comprise erratics belonging to several glacial episodes and are associated with preglacial tors that survived the Quaternary period owing to their protection under cold-based ice (Kleman and Borgstro¨m, 1990); 2. Parts of the Scandinavian blockfields which have been removed by Late Weichselian glacifluvial floods, as in the Tornetra¨sk area (northern Sweden), were not formed again by Holocene weathering that only induced a millimetric biogenic granular disintegration at the bedrock surface (Andre´, 1996).

Fig. 6. Holocene talus cones and lobate rock glaciers at Tempelfjord, central Spitsbergen, 78jN (photo by R. Coque).

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Fig. 7. Quartzitic felsenmeer developed on the plateaus of the Korok valley, Labrador (photo by A. Godard).

Therefore, as earlier suggested by Caine (1968) in Tasmania, many blockfields might represent the truncated roots of pre-Quaternary weathering profiles. Depending on their original depth in the regolith, they may contain or not clay minerals like kaolinite and associated gibbsite pointing to warm-climate preglacial conditions (Roaldset et al., 1982; Rea et al., 1996). The antiquity of blockfields is also supported by the coincidence of their distribution with that of Tertiary planation surfaces and associated torlike features, for instance in Norway. This hypothesis is probably applicable to different types of blockfields, including the stone runs of the Falkland islands (Rosenbaum, 1996) whose superficial openwork morphology masks reddish iron-rich roots (observ. Andre´ and Hall, December 16, 1999, and December 14, 2001). Therefore, many blockfields derive most probably from Tertiary chemical weathering and have been only reworked by glacial and/or periglacial processes. Apart from often comprising erratic blocks and till material, they happen to display specific minor-scale glacial features like perched blocks (blocs superpose´s and blocs tripodes), as shown by Sellier (1995) in Norway. Further periglacial reworking of blockfield material seems to depend mainly on

the abundance of fine-grained matrix that accounts for the occurrence of patterned ground such as stone circles. Overall, the Quaternary cold-climate conditions seem to have slightly modified the blockfields that owe their origin to chemical weathering rather than to frost shattering. Most of them are currently being subjected to biogenic microweathering under a widespread lichen cover, as in northern Labrador (Andre´, 1982). Richter denudation slopes, i.e. rectilinear slopes at 30– 35j cut in bedrock (Fig. 8), have been interpreted as the products of Quaternary frost shattering that erased cornices and other irregularities (e.g., Coque, 1977). This hypothesis is probably valid in particular contexts (e.g., rapid postglacial evolution of cirque mountain walls in frost-susceptible lithologies). However, in many cases, the position of Richter slopes within the slope system points to a pre-Quaternary origin. In Canada and Scandinavia, these rectilinear slopes are frequently located at the upper extremity of Tertiary planation surfaces, and they are dissected by perpendicular glacial troughs (e.g., Andre´, 1982, 1993; cf. Fig. 9). In the Torngat Mts (northern Labrador), glacial cirques are even cut within Richter slopes. This antiquity was also demonstrated in more southern areas. In the French Pyre´ne´es, for instance,

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Fig. 8. Richter denudation slope, Scheteligfjellet, NW Spitsbergen, 79jN.

gneiss Richter slopes are contemporaneous with Late Tertiary planation surfaces and are locally dissected by glacial cirques (Calvet, 2001). Moreover, in the Adriatic Apennines, the position of certain rectilinear

slopes suggests also a pre-Quaternary origin (Dufaure et al., 1989), and in southern Greece, the lower part of Richter slopes even happens to have been under marine Pliocene deposits (Dufaure, written commun.,

Fig. 9. Richter denudation slopes dissected by glacial troughs and cirques, based on field observations in West Spitsbergen and northern Labrador.

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October 2001). The ‘‘freeze –thaw panacea’’ (Hall, 1995) definitely cannot account for the genesis of all Richter slopes, not only because of their position within the slope system in many arctic and alpine areas, but also due to their ubiquity. Although widespread in polar areas, Richter slopes are also found in warm regions such as the Mauritanian Adrar and the Bushveld in South Africa (Daveau, 1967; Lageat, 1989). Most probably, the preexisting jointing of the bedrock due to tectonic stresses has been exploited by various processes to generate these rectilinear slopes.

4. Conclusion As stated by Matsuoka (2001) and Hall et al. (in press), there is an urgent need both for ‘‘bridging the gap between laboratory and field frost weathering’’ studies, and for diversifying the monitoring protocols. Most of them were designed to demonstrate the possibility of freeze – thaw mechanisms. The time has come to explore all the processes potentially involved in rock weathering, including the chemical, thermal and biological ones. Thus, the presence of salts should be systematically investigated, for it can play an important role in the initiation and development of alveolar weathering, as shown in the Antarctic Dry Valleys (e.g., Wellman and Wilson, 1965; Campbell and Claridge, 1987). Moreover, high-resolution rock temperature and moisture data recently collected in Antarctica (Hall, in progress) point to the possibility of a thermally driven landscape evolution similar to that found in hot deserts as already suggested (e.g., Hall, 1999; Hall and Andre´, 2001).

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More generally, the smokescreen of the presupposed frost-derived angular debris should be penetrated to investigate the ‘‘invisible world’’ of chemical reactions and microbiological life. The chemical reactions must be taken into account in the monitoring procedures and associated laboratory analyses as it was done in Alaska and North Sweden (Dixon et al., 1984, 2001). As to the importance of bacterial, fungal and lichenic biofilms, this has been amply demonstrated in Antarctica, Alaska and Iceland (e.g., Friedmann, 1982; Ascaso et al., 1990; Hall and Otte, 1990; Sun and Friedmann, 1999; Etienne, 2002). Rather than a trifling process, biogenic weathering should be considered as a crucial one (Etienne, 2002) in many cold areas where microorganisms occupy strategic endolithic biotopes (Golubic et al., 1981; see Fig. 10). Biogenic weathering operates often slowly, but is very important owing to its long-lasting action and its wide distribution. It affects not only the horizontal ice-scoured outcrops of polar shields, but also many vertical rockwalls that are too massive to be susceptible to frost shattering. Apart from refining and diversifying the protocols, it appears necessary to balance the sophisticated ongoing monitoring studies with a more historical approach of landform evolution based on geomorphological mapping. It must be kept in mind that the current processes operate after a series of different ones that are responsible for the formation of most of the observed geomorphic features, which are inherited. Widening the perspective, both in space and in time, should help both to assess the representativeness of the monitoring data collected in restricted sites and to avoid misinterpretations based on the uniformitarian approach.

Fig. 10. Strategic positions occupied by cyanobacteria in the upper part of outcrops (Golubic et al., 1981).

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