Géomorphologie : relief, processus, environnement, 2008, n° 4, p. 223-234

Paraglacial and paraperiglacial landsystems: concepts, temporal scales and spatial distribution Géosystèmes paraglaciaire et parapériglaciaire : concepts, échelles temporelles et distribution spatiale

Denis Mercier*

Abstract The Pleistocene Earth history has been characterized by major climatic fluctuations. During glacial periods, ice may have covered around 30 per cent of the Earth surface compared to approximately 10 per cent nowadays. With global change, polar environments and other montainous glacial environments of the world are presently undergoing the most important changes since the end of the Last Glacial Maximum and are experiencing paraglacial and paraperiglacial geomorphological readjustments. Paraglacial and para- periglacial landsystems consist of several subsystems including gravitational, fluvial, coastal, aeolian and lacustrine environments. Paraglacial and paraperiglacial landsystems can be analysed as open and complex landsystems characterized by energy, water and sed- iment fluxes and exchange with surrounding environments, especially with glacial and periglacial landsystems as inputs. Those cascading landsystems are likely to react to climate change because they rely on an ice-cold water stock (glacier and permafrost) that developed during a previous cold sequence (glaciation). The response of paraglacial and paraperiglacial systems to climatic forcing takes place over a long time span ranging from an immediate reaction to several millennia. The spatial limits of paraglacial and para- periglacial landsystems are inherently dependant on the time scale over which the system is analyzed. During the Pleistocene, glaciations widely affected the high latitudes and the high altitudes of the Earth and were followed by inherited paraglacial sequences. Glacier forelands in Arctic and alpine areas experience paraglacial processes with the present warming. The expected global warming for the twenty-first century will result in significant impacts on present glacier areas in mountains and could result in the appearance of new areas for paraglacial dynamics. In permafrost terrain, landscapes underwent a similar paraperiglacial geomorphological adjust- ment in mountainous, continental and coastal areas, with permafrost thaw-degradation and thermokarst processes. Key words: climate change, permafrost, thermokarst, Arctic, coastal system, river system, cascading system.

Résumé L’histoire pléistocène de la Terre est caractérisée par des fluctuations climatiques majeures. Pendant les périodes glaciaires, la Terre était recouverte jusqu’à 30 % de sa surface par de la glace, qui en occupe approximativement 10 % aujourd’hui. Avec le changement climatique, les environnements des régions polaires et les environnements des montagnes englacées connaissent actuellement la plus grande métamorphose depuis la fin du dernier maximum glaciaire. Ils subissent des réajustements géomorphologiques paraglaciaires et parapériglaciaires. Les géosystèmes paraglaciaire et parapériglaciaire sont définis par différents sous-systèmes : gravitaire, fluvial, littoral, éolien, lacustre. Les géosystèmes paraglaciaires et parapériglaciaires peuvent être analysés comme des systèmes complexes ouverts, traversés par des flux et des échanges d’énergie, d’eau et de sédiments, en relation avec la périphérie, notamment avec les géosystèmes glaciaire et périglaciaire comme entrées du système. Ces géosystèmes en cascade sont sensibles aux changements clima- tiques car ils dépendent d’un stock de glace (glacier et pergélisol) constitué au cours de la glaciation antérieure. La réponse des géosystèmes paraglaciaires et parapériglaciaires aux forçages climatiques dépend des échelles temporelles retenues pour l’analyse. Au cours du Pléistocène, les glaciations ont largement affecté les hautes latitudes et les hautes altitudes de la planète et ont été suivies par des séquences paraglaciaires héritées. Avec le réchauffement contemporain, les marges glaciaires de l’Arctique et des montagnes alpines correspondent aux espaces des dynamiques paraglaciaires actives. Avec le réchauffement envisagé pour le XXIe siècle, les espaces de montagne actuellement englacés seront affectés et deviendront les espaces potentiels pour la dynamique paraglaciaire. Les espaces qui furent affectés par un pergélisol ont connu un réajustement géomorphologique parapériglaciaire avec la dégradation du pergélisol et les processus thermokarstiques, dans les espaces montagneux, continentaux et littoraux. Mots clés : changement climatique, pergélisol, thermokarst, Arctique, système littoral, système fluvial, système en cascade.

* Université de Nantes, laboratoire Géolittomer, CNRS - UMR 6554 LETG, campus du Tertre, BP 81227, 44312 Nantes cedex 3. Courriel : [email protected] Denis Mercier

Version française abrégée affecter les entrées des géosystèmes (flux d’énergie ther- mique, flux liquides et solides). Directement, chaque chan- Actuellement, environ 10 % de la surface terrestre sont re- gement climatique affecte certaines variables endogènes des couverts de glaciers. Au cours des séquences froides du géosystèmes comme la vitesse et la nature des processus Pléistocène, cette surface englacée s’est élevée à 30 %. (ruissellement) ou le recouvrement végétal. Les changements Selon l’IPCC (Solomon et al., 2007), les effets du réchauffe- climatiques interviennent sur les géosystèmes paraglaciaires ment climatique seront notablement plus importants dans les et parapériglaciaires selon différentes échelles temporelles. À régions polaires, notamment en Arctique, que dans les autres l’échelle des interglaciaires (Éémien, Holocène), les phases régions de la planète. De ce fait les environnements englacés paraglaciaire et parapériglaciaire s’expriment au cours de et leurs périphéries froides seront affectés et connaissent dizaines de milliers d’années. La période post-petit âge gla- déjà, depuis la fin du petit âge glaciaire, le plus grand ré- ciaire offre une échelle centennale à la séquence paragla- ajustement géomorphologique après celui de la fin de la der- ciaire et parapériglaciaire. Dans le cadre du réchauffement nière grande glaciation. Ces réajustements géomorpholo- climatique contemporain, l’échelle décennale se surimpose à giques peuvent être qualifiés de paraglaciaires et parapéri- l’échelle centennale. Depuis un siècle, une phase d’accélé- glaciaires. Ils affectent des environnements froids et leur ap- ration du réchauffement climatique (1980–2007) a succédé à proche peut se faire sous l’angle des géosystèmes face aux une phase de refroidissement (1940–1970). L’intensité des changements climatiques. changements et leur durée n’étant pas les mêmes, il est pos- Le concept de paraglaciaire a été créé par M. Church et sible de définir des réactions plus ou moins importantes des J. Ryder (1972) pour définir les processus non glaciaires géosystèmes paraglaciaires et parapériglaciaires. qui sont directement conditionnés par la glaciation et une La réponse des dynamiques paraglaciaires et parapériglai- période au cours de laquelle les processus paraglaciaires caires aux forçages climatiques peut être instantanée ou dif- sont particulièrement efficaces. En 2002, C. Ballantyne a férée de plusieurs millénaires. La réponse instantanée s’ex- proposé une définition plus large du concept de paragla- prime par exemple dans le cadre du sous-système fluvial (jö- ciaire et a suggéré de l’étendre aux accumulations sédi- kulhlaups ; Roussel, 2008 ; Étienne et al., 2008). Selon un mentaires, aux modelés, aux géosystèmes et aux paysages pas de temps décennal, le retrait des glaciers laisse aux pieds qui sont directement conditionnés par les glaciations et les des versants des moraines latérales, rapidement remaniées déglaciations. C. Ballantyne (2005) propose six géosys- par des glissements et des coulées de débris activés par le tèmes paraglaciaires : versant rocheux, dépôts de versant, ruissellement de fonte de la glace morte au sein du dépôt. marges glaciaires, systèmes fluviaux, lacustres et côtiers, Cette dynamique paraglaciaire s’achève rapidement après la chacun présentant des interrelations. L’analyse du géosystème disparition de la glace morte en quelques décennies (Curry, peut se faire selon une approche consacrée aux formes (fig. 1) 1999 ; Mercier, 2001). Les processus thermokarstiques affec- ou aux processus, notamment ceux qui agissent en cascade et tant les littoraux arctiques répondent également à un pas de aux transferts de flux (fig. 2). Parallèlement au géosystème temps similaire (Hill et al., 1994 ; Lantuit et Pollard, 2008). paraglaciaire, nous proposons d’utiliser le concept de para- Au cours d’une période centennale, les évolutions du sous- périglaciaire pour désigner les processus à la surface de la système fluvial se traduisent par des reculs de berges associés Terre, les accumulations sédimentaires, les modelés, les sys- aux processus thermokarstiques et par un accroissement des tèmes et les paysages qui sont directement conditionnés par la flux sédimentaires exportés jusqu’à la mer. Au cours d’une dégradation du pergélisol. Le géosystème parapériglaciaire période plurimillénnaire, la décohésion des parois est l’une peut également être analysé comme un système de processus des dynamiques les plus efficaces sur le plan morphogénique en cascade (fig. 3). (Ballantyne, 2002 ; Cossart et al., 2008). Enfin, à l’échelle de Les différentes utilisations du concept de parapériglaciai- l’ensemble du Pléistocène, la sédimentation offshore au sein re dans la littérature scientifique sont recensées. Elles dési- du sous-système marin illustre parfaitement les alternances gnent le plus souvent des périodes de transition dans les es- des séquences glaciaires et parapériglaciaires (Ottesen et al., paces continentaux, de montagnes et sur les littoraux. Les 2005a et 2005b ; Rise et al., 2005). dépôts weichseliens de haut de falaise (head) constituent les La répartition spatiale des dynamiques paraglaciaires et principaux et souvent les seuls stocks sédimentaires actuels parapériglaciaires dépend des échelles temporelles. Une pour le rechargement des plages. Ils sont paraglaciaires carte des espaces ayant subi une ou plusieurs séquences pa- dans le cas des littoraux ayant été englacés comme en Gran- raglaciaires et parapériglaciaires au cours du Pléistocène de-Bretagne (Ballantyne et Harris, 1994) et parapérigla- est proposée (fig. 5). Elle représente les espaces de l’hémi- ciaires dans le cas des espaces qui ne l’ont pas été (côtes sphère nord ayant été recouverts d’un inlandsis et les es- galiciennes, Blanco-Chao et al., 2007 ; ou normandes, paces montagneux englacés y compris les espaces mari- fig. 4). Les milieux paraglaciaires et parapériglaciaires sont times qui ont enregistré les sédiments issus de la dynamique soumis à des changements climatiques dont le rythme et paraglaciaire. Actuellement, les espaces paraglaciaires ac- l’intensité les transforment en « forçages ». Ces géosys- tifs correspondent aux marges des glaciers en retrait. Les tèmes sont d’autant plus sensibles aux changements clima- espaces de la dynamique paraglaciaire potentielle sont ceux tiques qu’ils s’expriment aux dépens d’un stock d’eau gelée qui sont susceptibles d’être affectés par le réchauffement (glacier et pergélisol) constitué pendant une glaciation an- climatique pronostiqué au cours du XXIe siècle. Les espaces térieure. Indirectement, les changements climatiques vont parapériglaciaires se localisent à la périphérie des zones

224 Géomorphologie : relief, processus, environnement, 2008, n° 4, p. 223-234 Paraglacial and paraperiglacial landsystems: concepts, temporal creates and spatial distribution englacées et correspondent aux limites de l’extension maxi- cribe postglacial dynamics and landforms in Scotland. A male du pergélisol au Pléistocène. On retrouve tous les es- century earlier, several geomorphological studies described paces continentaux en Amérique du Nord, Europe et Asie, post-Little Ice Age readjustment and used different expres- ayant connu un ajustement morphogénique à l’Holocène sions such as « torrential era » (Surell, 1841), « diluvial (Van Vliet-Lanoë et Lysitsyna, 2001). Durant le Quaternai- period » (Martins, 1867), « alluvial fan period » (Girardin, re, les mêmes espaces peuvent avoir connu à plusieurs re- 1910) as paraglacial synonyms (Mercier, 2007). Recently, prises des phases paraglaciaires et parapériglaciaires qui C. Ballantyne (2002) proposed a new and larger definition se sont surimposées. of the paraglacial concept: « non-glacial earth-surface pro- cesses, sediment accumulations, landforms, landsystems Introduction and landscapes that are directly conditioned by glaciation and deglaciation ». The past two million years of Earth’s history have been cha- Applications of landsystems concept to assessments of racterized by major climatic fluctuations. During glacial per- glaciated terrain have been proposed in a holistic point of iods, glacier ice covered up to about thirty per cent of the view by D. Evans (2005). In his synthesis of paraglacial Earth surface. Glacier ice presently covers approximately ten geomorphology, C. Ballantyne (2005) recognized six ‘para- per cent or almost 16 million km2 of the Earth’s surface. The glacial landsystems’: rock slopes, drift-mantled slope, Antarctic (13.5 million km2) and Greenland (2 million km2) glacier forelands, alluvial, lacustrine and coastal systems, ice sheets form the bulk of those. Only three per cent (or each containing a variety of paraglacial landforms and sedi- 500,000 km2) are small glaciers located in high latitudes and ment facies. The paraglacial landsystem should be composed mountainous regions (Benn and Evans, 1998; Van Vliet- of several sub-systems characterized by gravity, fluvial, Lanöe, 2005; Francou and Vincent, 2007). Polar environ- coastal, aeolian, lacustrine, or offshore processes (Mercier, ments and other montainous glacial environments around the 2008). A paraglacial landsystem, like other geomorphologi- world presently experience the most important changes since cal systems, should be analysed by three kinds of approach the end of the Last Glacial Maximum during the Pleistocene. (Huggett, 2007). Firstly, form systems are defined as « sets In its latest report (Solomon et al., 2007) the Intergovern- of form variables that are deemed to interrelate in a mea- mental Panel on Climate Change scientific committee confir- ningful way in terms of system origin or system function » med its previous scenarios for Polar Regions. The effect of (Huggett, 2007). They could be measured and mapped global change might be amplified in the Arctic due to feed- without connection between the processes and the forms. backs between cryospheric systems (glacier extent, snow Figure 1 presents a schematic paraglacial landforms system. cover, sea ice variability, permafrost), land system (tundra, Secondly, process systems, which are also called cascading soils, hydrology), atmospheric systems and ocean interac- or flow systems, are defined by A. Strahler (1980) as tions. Both glacial and non-glacial (periglacial) cold-climate « interconnected pathways of transport of energy and matter regions are severely affected by climate warming. Since the or both, together with such storages of energy and matter as end of the Little Ice Age, glacial environments are experien- may be required ». C. Ballantyne (2002) proposed a sim- cing a climatic crisis leading to a paraglacial geomorphologi- plified paraglacial sediment cascade showing the principal cal readjusment; and periglacial regions, characterized by per- primary and secondary sediment stores and main sediment mafrost, are experiencing what could be defined as a ‘parape- transfer processes (fig. 2). Thirdly, process-form systems, riglacial’ period. The main aim of this paper is to suggest a also styled process-response systems, are defined as « an new definition of the term ‘Paraglacial landsystem’, proposed energy-flow system linked to a form system in such a way by C. Ballantyne (2005), and of the term ‘Paraperiglacial’, to that system processes may alter the system form and, in establish the relationship between temporal scales and para- turn, the changed system form alters the system processes » glacial and paraperiglacial landsystems, and to show the spa- (Huggett, 2007). The systems approach allows us to study tial distribution of present and palaeo-paraglacial and parape- the sub-systems of the paraglacial landsystem in an integra- riglacial dynamics and landscapes. ted way and to focus on the fluxes of energy and sediments as a response to climate change. The paraglacial and paraperiglacial Following the definition of paraglacial by C. Ballantyne landsystem (2002), it is suggested to use the concept of paraperiglacial to define: « earth-surface processes, sediment accumula- Etymologically, the term « paraglacial » means « next to tions, landforms, landsystems and landscapes that are di- the ice », because this word consists of the Greek prefix rectly conditioned by permafrost thaw-degradation ». This « para », next to, and of the Latin « glacies », ice. This paraperiglacial concept could be used in all periglacial envi- term was used by J. Ryder (1971a, b) to describe alluvial ronments affected by climate change at both local and glo- fans deposited during the Late Wisconsin deglaciation in bal scales. Processes like meltwater flow, fluvial reworking, British Columbia. M. Church and J. Ryder (1972) formali- runoff, debris flow, thermokarst thaw and collapse can be sed the concept ‘paraglacial’ to define nonglacial processes assimilated into paraperiglacial processes, and several forms that are directly conditioned by glaciation and as a period of mass movements, thermokarst features and deposits can over which paraglacial processes operate. Previously, the be considered as paraperiglacial sediment stores (fig. 3). term paraglacial had been used by A. Godard (1965) to des- During source-to-sink transport, paraperiglacial processes

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Fig. 1 – Schematic paraglacial land- forms system. Fig. 1 – Représentation schématique des formes du système paraglaciaire. succeed to periglacial processes and preceed azonal processes. The ‘paraperiglacial’ concept defines processes associated with perma- frost thaw-degradation, a period over which paraperiglacial pro- cesses operate and areas in cold non-glacial environments affected by those changes. Russian Quaternary scientists were the first to use the term para- periglacial to describe a period cha- racterized by climate warming du- ring interglacial stages (Velichko and Timireva, 1995), or to describe areas affected by interglacial varia- tions in the vegetation cover resul- ting from climate change (Gribchen- ko and Kurenkova, 1997). They used this term to describe periglacial land- systems affected by warming and the consequent rapid degradation of per- mafrost, which resulted in instability of the land surface, thermokarst, and expansion of wetlands. They also explain the disappearance of the Mammoth during this period by these paraperiglacial environmental changes (Velichko and Zelison, 2005). The same reason also seemed to be the crucial factor that caused the abandonment of traditional sett- lement areas by Palaeolithic human groups (Gribchenko and Kurenkova, 1997). Previously, in another seman- tic way, A. Corte (1983, 1986) used the terms ‘paraperiglacial’, ‘para- geocryogenic’ and ‘paraglacial’ to describe the spatial distribution of processes and landforms in the Cen- tral Andes. He used ‘paraglacial’ to describe all facies of covered ice. The term ‘para-periglacial’ or ‘para-

Fig. 2 – Simplified paraglacial sedi- ment cascade (After Ballantyne, 2002. Reproduced by permission of Elsevier). Fig. 2 – Schéma simplifié du transfert sédimentaire en cascade en milieu paraglaciaire. (d’après Ballantyne, 2002. Reproduit avec la permission d’Elsevier).

226 Géomorphologie : relief, processus, environnement, 2008, n° 4, p. 223-234 Paraglacial and paraperiglacial landsystems: concepts, temporal creates and spatial distribution

Fig. 3 – Simplified paraperiglacial sediment cascade. Fig. 3 – Schéma simplifié du transfert sédimentaire en cascade en milieu parapériglaciaire.

today in the form of a positive re- lationship between rock hardness and intertidal elevation. In this context, most coasts on both sides the Atlantic Ocean under- went a paraperiglacial evolution during the Holocene and during other interglacial eras, such as the Eemian. For instance, all French coasts covered by continental de- posits during the Weichselian period could be considered to be para-periglacial coasts (fig. 4). Those periglacial deposits (head) were exhumed during sea-level transgression and constitute the main, and often the only, source of sediments for coarse-grained beaches, similarly to till deposits for paraglacial coasts in the Arctic and around Weichselian ice-sheets (e.g., in South Baltic sea). But in another spatial context around the coasts of lowland Britain, such de- posits called ‘head’ are reinterpre- ted as evidence of paraglacial conditions because those deposits represent meltout tills that were reworked by solifluction and mudflow, and were active during geocryogenic’ was used to define the seasonal ground-free- a period of immediate postglacial readjustment (Wright, zing region. This term was defined in a letter of 20 Septem- 1991; Ballantyne and Harris, 1994). For D. Brunsden (2001) ber 1982 entitled ‘the paraperiglacial environment’ by Ko- the paraglacial and para-periglacial supply of sediment to walkowski as follows: ‘a temperate climate zone with per- many beaches became depleted due to sea-level rise in the iodic freezing and thawing of soils and waste, seasonal snow late Holocene. In mountainous areas, G. Ibañez-Palacios and underground ice’. In the coastal sub-system, the term and A. Ahumada (2006) used the term ‘paraperiglacial’ as a para-periglacial is used by R. Blanco-Chao et al. (2007), synonym of ‘parageocryogenic’ for a zone, from 2 000 m following the definition of paraglacial coasts by D. Forbes to 4 000 m a.s.l., on the Eastern slope of Aconquija Range, and J. Syvitski (1994) to describe Holocene conditions on characterized by seasonal ground freezing below the lowest shore platforms in Galicia and the former and continuing in- termini of rock glaciers. C. Le Cœur (2007) described post fluence of periglacial and fluvio-nival deposits on coastal Little Ice Age rock glacier evolution in the Alps, in the evolution and dynamics. They proposed a model to describe Cerces massif, and used the term paraperiglacial to charac- the recent evolution of what they called ‘a para-periglacial terize a dynamic associated with runoff, which reworks system’ (Blanco-Chao et al., 2007). In para-periglacial sys- parts of rock glacier deposits and contributes to subsequent tems, the relationship between intertidal rock strengh and landform modification. The term ‘paraperiglacial’ is propo- tidal level may be a function of the occurrence, and nature sed by Le Coeur to indicate also the sequence of evolution of, continental slope deposits that impinged on the coastal of a landform built by periglacial dynamics and non-frost domain. With rising Holocene sea level, erosion and retreat specific processes, which henceforth affect the surface of of sedimentary cliff deposits caused abrasion zones to mi- the landform. This dynamic cannot evacuate the totality of grate landwards, as the coarse sediments were released from the accumulated sediment in cold conditions, but it reshapes the deposits; the effect of this migration may be preserved it and can contribute to its fossilization under slope deposits.

Géomorphologie : relief, processus, environnement, 2008, n° 4, p. 223-234 227 Denis Mercier

Fig. 4 – Paraperiglacial coast in Normandy, Ecalgrain bay. A: basement with Cambrian sandstones; B: cliffs cut in the continental periglacial deposits (head), Pleistocene solifluction deposits that constitute the main stock of available sediments for the beach of the fore- ground (C) (photo: D. Mercier, 2002). Fig. 4 – Côte parapériglaciaire en Normandie, Baie d’Ecalgrain. A : socle constitué de grès cambriens ; B : falaises taillées dans les dépôts périglaciaires continentaux (head), dépôts de solifluxion du Pleistocène formant l’essentiel du stock de sédiments disponibles pour la plage du premier plan (C) (photo : D. Mercier, 2002).

Temporal scales of the paraglacial intensity of the changes and their duration, important reac- and paraperiglacial landsystems tions of the paraglacial and paraperiglacial landsystems can be determined. By definition, the paraglacial and paraperiglacial landsys- The response of the systems to forcing takes place over a tems are active over a temporal sequence, which corresponds range of time spans ranging from an immediate reaction to to a morphogenic readjustement. Traditionally, it fits in time several millennia. Several examples are as follows. Firstly, between the cold period and the period characterized by ‘nor- immediate response could be analysed for fluvial sub-sys- mal’ or common slope to river erosion. The paraglacial phase tem. Jökulhlaups are crises of strong intensity, which cor- thus possesses a limited, calculable life expectancy from the respond typically to a forcing of sub-glacial volcanic origin quantification of the deposited volumes, the rates of denuda- and are often observed in Iceland (Kristmannsdóttir et al, tion and the dating of the beginning of the phase. It is 1999; Russell and Knudsen, 1999; Knudsen et al, 2001; Ste- characterized by a curve of sediment supply corresponding fansdóttir and Gislason, 2005). Jökulhlaups cause sandur to a negative exponential function (Ballantyne, 2003). The aggradation, which may be eroded within few years (Smith life time of the paraglacial and paraperiglacial sequence et al, 2006; Roussel, 2008). These floods can also be the re- depends on the amount of sediment stock to be reshaped; on sult of break of the ice dam (Björnsson, 1992; Étienne et al., the rate of the processes and also on the climatic parameters 2008). On the other hand, some debris flows in mountain and the geographical location of the catchment; on the post- environment with glacier retreat are associated with glacial glacial vegetation cover; on the size, on the orographic and lake outbursts and usually occur during the summer months on the geologic nature of catchment, in which the paraglacial when they coincide with intense glacier ablation and melt- and paraperiglacial processes operate. After exhaustion of the water production (Passmore et al., 2008). Debris flows in sedimentary stocks, the paraglacial and the paraperiglacial this high altitude environment, with the availability of un- sequences end. The paraglacial and paraperiglacial landsys- consolidated sediments, had been considered as an impor- tems are likely to react to climate change because they rely tant component of the paraglacial response to glacier retreat on an ice-cold water stock (glacier and permafrost) generated since the end of the Little Ice Age. Secondly, glacier retreat during a previous glaciation. Climate change will affect the leaves moraines, quickly reshaped by glidings and poured external inputs (thermal flow of energy; water and solids by fragments activated by streaming of melting of the dead inflow) and affects the landystems through distinct endoge- ice within the deposit. This dynamic is paraglacial by natu- nous variables as the rate and the nature of the processes or re and ends within decades following the disappearance of the vegetation cover. Climate change acts upon the landys- dead-ice (Curry, 1999; Curry and Ballantyne, 1999; Mer- tems at various temporal scales. During interglacial periods cier, 1997, 2001). For paraperiglacial landsystem, thermo- (Eemian, Holocene), the paraglacial and paraperiglacial per- karst landscapes or retrogressive thaw-slump on coastal iods of tens of thousand years. During the post-Little Ice Age zone (Hill et al., 1994; Lantuit and Pollard, 2008) responds period, the sequence is of the order of the century and is to climate warming on a same decadal scale. Thirdly, the superimposed on the Holocene, while the LIA constituted a most prominent modification of the relief at one hundred period of slowing down of the paraglacial Holocene dyna- years scale is probably associated with fluvial sub-systems. mics. At a consequence of recent abrupt climate changes, Progradation of seaward downstream to the conquering paraglacial and paraperiglacial activity are visible at the ten- river systems were quantified in Svalbard (Mercier and Laf- year scale and are superimposed on the century scale. A fly, 2005; Roussel, 2005). A predictive model built from period of acceleration of global warming (1980-2007) follo- data from forty-six rivers of the Arctic showed that for a wed a period of cooling (1940-1970). Despite differences in 2 °C rise rivers would increase their sedimentary discharge

228 Géomorphologie : relief, processus, environnement, 2008, n° 4, p. 223-234 Paraglacial and paraperiglacial landsystems: concepts, temporal creates and spatial distribution by 22% (Syvitski, 2002). B. Peterson et al. (2002) studied vity during the Holocene deglaciation. In high altitudes areas the six largest Eurasian arctic rivers (including the Lena) (Cantabrique, Pyrénées, Alps, Caucasus, Pamir, Tien Shan, and calculated an increase in annual average rate of dischar- TransBaikalian mountains…), mountainous areas and pie- ge of 2.0 ± 0.7 km3, which corresponds to an approximate monts were completely ice-covered or semi-continuously co- 7% increase. Recent climate warming induces the propaga- vered by ice caps and extensive glacier complexes. Paragla- tion of a thawing line within the frozen riverbank of the cial sequences also affected coastal areas (e.g., south Baltic Lena River and this paraperiglacial process produces niches, coast) and continental margins with offshore deposits (e.g., which contribute to the disequilibrium of the bank by ther- North sea, Scandinavian continental shelf, Barents sea, Kara mokarstic subsidence and subsequent large slumps along the sea, Laptev sea). All of these areas correspond nowadays to river-banks (Walker, 1983; Costard et al., 2007). Since the landscapes of inherited paraglacial deposits. Contemporary end of the Little Ice Age, palsa degradation in subartic is in- global warming affects in particular glacier margins that are fluenced by several regional and local factors including in- known to have readvanced during the Little Ice Age. All gla- crease air temperature, changes in depth of snow cover, cial forelands in Arctic and alpine areas are experiencing pa- water level fluctuations in the river (Vallée and Payette, raglacial processes under present warming. The expected 2007). Fourthly, one of the consequences of the deglaciation global warming for the twenty-first century will result in si- is the exposure of formerly englaciated mountain walls. The gnificant impacts on present glacial areas in mountains and disappearance of the ice induces the occurrence of major gli- could result in the appearance of new areas for paraglacial dings, collapses or massive collapses (0.25-3.0 km2), often dynamics. representing lethal threats to infrastructure and human sett- In the vicinity of glaciers and in non-glacial cold envi- lements. They can also introduce slow progressive deforma- ronments, permafrost landscapes underwent a similar geo- tions of hillside, either still the periodic adjustments of hill- morphological readjustment that can be considered as para- sides by falls or glide of small sizes (André, 1997; Ballanty- periglacial. One quarter of the earth’s land surface current- ne, 2008; Cossart et al., 2008; Iturrizaga, 2008; Jarman, ly experiences periglacial conditions. During colder periods 2006; Sellier, 2008). This paraglacial dynamics can occur of the Pleistocene, an additional one fifth of continental within a few millennia after deglaciation as evidenced by areas were affected by periglacial conditions associated cosmonucleous datings (Ballantyne et al., 2008). Finally, with the presence of continuous permafrost around ice- the analysis of offshore sedimentation in the marine sub- sheets in North America, Europe and Russia (Van Vliet- system provides proxies for the reconstitution of glacial and Lanoë and Lysitsyna, 2001; French, 2007). At the time scale paraglacial sequences and rates of sedimentation over the of the Quaternary, periglacial environments roughly coinci- last three million years (Ottesen et al., 2005a and 2005b). At ded with various advances and retreats of continental ice least 1000 m of sediments accumulated on the Scandinavian sheets. During the Holocene period and the retreat of North margin during this period of time (Rise et al., 2005). American, Scandinavian and Russian ice sheets, periglacial conditions became predominant. With the warming of high Spatial distribution latitude environments, periglacial areas also experience pa- of paraglacial and paraperiglacial raperiglacial period, which could be superimposed on inhe- landsystems rited paraglacial landscapes. An example could be retro- gressive thaw slump activity on Herschel Island in the Ca- The spatial limits of the system are inherently dependant nadian Arctic, which is due to paraperiglacial processes on the time scale at which the system is considered. Com- (warming permafrost, increasing active layer depths and monly glacial and interglacial periods are known to be pe- thermokasrt activity) and environmental changes (sea level culiar features of the Pleistocene. According to several au- rise, reduction in sea ice extent and duration, increasing thors, the maximum ice-sheet extent took place during this storms impacts) affecting inherited paraglacial deposits period (fig. 5) (Andersen and Borns Jr., 1997; Grosswald, (Lantuit and Pollard, 2008). 1998; Ehlers and Gibbard, 2004a and 2004b; Svendsen et al., 2004). During the Pleistocene, glaciations widely affected Conclusions the high latitudes and the high altitudes of the Earth and were followed by inherited paraglacial sequences. The Holocene The paraglacial and paraperiglacial landsystems are com- contains an active paraglacial period, which follows the last plex open cascading systems very sensitive to climate chan- cold Pleistocene period. The extent of the ice-sheets during ge. The responses of the paraglacial landsystem to climate the cold periods of the Pleistocene was characterized by pa- change span over time scales ranging from the immediate raglacial activity during interglacial periods. The great gla- reaction to the million years. The life expectation of the pa- ciations and post-glacial sequences were times of global raglacial landsystem is certainly limited, but can be prolon- reorganization in the Earth’s hydrological system. Glacia- ged for several centuries or millenniums in areas of sedi- tions and paraglacial sequences caused profound changes in ment storage (Cossart and Fort, 2008). A paraglacial period all elements of the system (glaciers, rivers, lakes, oceans, at- follows a glacial period and would logically be followed by mospheric moisture). On the continental areas, the main part a period of ‘normal’ erosion. Due to the brief duration of in- of North America, Scandinavia, and northern Russia were terglacial periods however, the paraglacial phases cannot be covered by ice-sheets and became areas of paraglacial acti- finished totally before the beginning of the following gla-

Géomorphologie : relief, processus, environnement, 2008, n° 4, p. 223-234 229 Denis Mercier

Fig. 5 – Spatial distribution of palaeo- and active paraglacial and paraperiglacial landsystems and landscapes. 1: present ice-sheet; 2: present alpine glacier, potential paraglacial area in the future due to global warming; 3: paraglacial area during Holocene deglaciation; 4: paraperiglacial area during Holocene warming; 5: the maximum ice-sheet extent during late Cenozoic glaciations; 6: the maximum perma- frost extent during Pleistocene period. Fig. 5 – Répartition spatiale des systèmes et des paysages paraglaciaires et parapériglaciaires hérités et actifs. 1 : inlandsis actuel ; 2 : glaciers de montagne actuels, espace potentiel du système morphogénique paraglaciaire en relation avec le réchauffement planétaire ; 3 : milieu paraglaciaire au cours de la déglaciation holocène ; 4 : milieu parapériglaciaire au cours du réchauffement holocène ; 5 : extension maximale des inslandsis au cours des glaciations du Cénozoïque ; 6 : extension maximale du pergélisol au cours du Pléistocène.

ciation. Climate change leads to paraglacial and paraperi- fected by a paraperiglacial crisis. The concepts of paragla- glacial geomorphological adjustment. Areas unglaciated cial and paraperiglacial periods provide a useful framework during the Pleistocene underwent a glacial period, then a for reconstruction of glaciated and periglacial landscapes paraglacial crisis, followed by a periglacial phase now af- evolution.

230 Géomorphologie : relief, processus, environnement, 2008, n° 4, p. 223-234 Paraglacial and paraperiglacial landsystems: concepts, temporal creates and spatial distribution

Acknowledgments cesses and landscape change. Britain in the last 1000 years. This paper is a synthesis of several years of scientific Blackwell, 32-60. research on the field in polar and subpolar environments Church M., Ryder J.M. (1972) – Paraglacial sedimentation: and discussions with colleagues. I would like to acknowled- consideration of fluvial processes conditioned by glaciation. ge M.-F. André, C.K. Ballantyne, É. Cossart, S. Étienne, Geological Society of America Bulletin 83, 3059-3072. B. Etzelmüller, T. Feuillet, M. Fort, L. Ménanteau, Corte A.E. (1983) – Los conceptos: geocriogenico – parageocrio- R. Neboit-Guilhot, D. Laffly, C. Le Cœur, J.-P. Peulvast, genico y glacial – paraglacial en los Andes Centrales de G. Rachlewicz, E. Roussel, D. Sellier for their stimulating Argentina, Latitud 30°. Anales 83 Instituto Argentino de Nivolo- and fruitful geomorphological discussions. Field investiga- gia y Glaciologia, 48-61. tions were supported by the CNRS, through the GDR 3062 Corte A.E. (1986) – Delimitation of geocryogenic (periglacial) ‘Mutations polaires’; GDR 49 ‘Recherches arctiques’; regions and associated geomorphic belts at 33° S.L. Andes of Géolab UMR 6042 (Clermont-Ferrand) and Géolittomer Mendoza, Argentina. Biuletyn peryglacjalny 31, 31-34. UMR 6554 LETG (Nantes). The French Polar Institute Cossart E., Braucher R., Fort M., Bourlès D.L., Carcaillet J. Paul-Émile Victor (IPEV) had supported project n°400 (2008) – Slope instability in relation to glacial debuttressing in ‘geomorphoclim’. I would warmly acknowledge E. Cooper alpine areas (Upper Durance catchment, southeastern France): from the Univesity of Tromsø, H. Lantuit from Alfred Wege- evidence from field data and 10Be cosmic ray exposure ages. ner Institut - Potsdam, and A. 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