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GEOMORPHOLOGIE 4-2008 BIS:Maquette Geomorpho

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GEOMORPHOLOGIE 4-2008 BIS:Maquette Geomorpho 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,
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