Paraglacial and paraperiglacial landsystems: concepts, temporal scales and spatial distribution
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 paraperiglacial 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 sediment 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 paraperiglacial 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 adjustment in mountainous, continental and coastal areas, with permafrost thaw-degradation and thermokarst processes.
Mots-clés :système fluvial, changement climatique, pergélisol, thermokarst, Arctique, système littoral, système en cascade
Keywords :climate change, permafrost, thermokarst, Arctic, coastal system, river system, cascading system
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Article soumis le 21 juillet 2008, accepté le 3 décembre 2008
This article has been published in open access since 01 January 2011.
This paper is a synthesis of several years of scientific research on the field in polar and subpolar environments and discussions with colleagues. I would like to acknowledge M.-F. André, C.K. Ballantyne, É. Cossart, S. Étienne, B. Etzelmüller, T. Feuillet, M. Fort, L. Ménanteau, R. Neboit-Guilhot, D. Laffly, C. Le Cœur, J.-P. Peulvast, G. Rachlewicz, E. Roussel, D. Sellier for their stimulating and fruitful geomorphological discussions. Field investigations were supported by the CNRS, through the GDR 3062 “Mutations polaires”; GDR 49 “Recherches arctiques”; Géolab UMR 6042 (Clermont-Ferrand) and Géolittomer UMR 6554 LETG (Nantes). The French Polar Institute Paul-Émile Victor (IPEV) had supported project n°400 “geomorphoclim”. I would warmly acknowledge E. Cooper from the Univesity of Tromsø, H. Lantuit from Alfred Wegener Institut - Potsdam, and A. Héquette from the University of Dunkerque, who carefully edited the English manuscript, F. Bonnaud from the University of Sorbonne and A. Dubois from the University of Nantes, who redraw figures one and five. Thanks are also due to Professors J. Dixon, K. Hall and J.-C. Thouret for painstaking and constructive reviews.
1The past two million years of Earth’s history have been characterized by major climatic fluctuations. During glacial periods, glacier ice covered up to about thirty per cent of the Earth surface. Glacier ice presently covers approximately ten per cent or almost 16 million km2 of the Earth’s surface. The Antarctic (13,5 million km2) and Greenland (2 million km2) ice sheets form the bulk of those. Only three per cent (or 500,000 km2) are small glaciers located in high latitudes and mountainous regions (Benn and Evans, 1998; Van Vliet-Lanöe, 2005; Francou and Vincent, 2007). Polar environments and other montainous glacial environments around the world presently experience the most important changes since the end of the Last Glacial Maximum during the Pleistocene. In its latest report (Solomon et al., 2007) the Intergovernmental Panel on Climate Change scientific committee confirmed its previous scenarios for Polar Regions. The effect of global change might be amplified in the Arctic due to feedbacks between cryospheric systems (glacier extent, snow cover, sea ice variability, permafrost), land system (tundra, soils, hydrology), atmospheric systems and ocean interactions. Both glacial and non-glacial (periglacial) cold-climate regions are severely affected by climate warming. Since the end of the Little Ice Age, glacial environments are experiencing a climatic crisis leading to a paraglacial geomorphological readjusment; and periglacial regions, characterized by permafrost, are experiencing what could be defined as a “paraperiglacial” period. The main aim of this paper is to suggest a new definition of the term “Paraglacial landsystem”, proposed by C. Ballantyne (2005), and of the term “Paraperiglacial”, to establish the relationship between temporal scales and paraglacial and paraperiglacial landsystems, and to show the spatial distribution of present and palaeo-paraglacial and paraperiglacial dynamics and landscapes.
2Etymologically, the term « paraglacial » means « next to the ice », because this word consists of the Greek prefix « para », next to, and of the Latin « glacies », ice. This term was used by J. Ryder (1971a, b) to describe alluvial fans deposited during the Late Wisconsin deglaciation in British Columbia. M. Church and J. Ryder (1972) formalised the concept “paraglacial” to define nonglacial processes that are directly conditioned by glaciation and as a period over which paraglacial processes operate. Previously, the term paraglacial had been used by A. Godard (1965) to describe postglacial dynamics and landforms in Scotland. A century earlier, several geomorphological studies described post-Little Ice Age readjustment and used different expressions such as « torrential era » (Surell, 1841), « diluvial period » (Martins, 1867), « alluvial fan period » (Girardin, 1910) as paraglacial synonyms (Mercier, 2007). Recently, C. Ballantyne (2002) proposed a new and larger definition of the paraglacial concept: « non-glacial earth-surface processes, sediment accumulations, landforms, landsystems and landscapes that are directly conditioned by glaciation and deglaciation ».
3Applications of landsystems concept to assessments of glaciated terrain have been proposed in a holistic point of view by D. Evans (2005). In his synthesis of paraglacial geomorphology, C. Ballantyne (2005) recognized six “paraglacial landsystems”: rock slopes, drift-mantled slope, glacier forelands, alluvial, lacustrine and coastal systems, each containing a variety of paraglacial landforms and sediment facies. The paraglacial landsystem should be composed of several sub-systems characterized by gravity, fluvial, coastal, aeolian, lacustrine, or offshore processes (Mercier, 2008). A paraglacial landsystem, like other geomorphological systems, should be analysed by three kinds of approach (Huggett, 2007). Firstly, form systems are defined as « sets of form variables that are deemed to interrelate in a meaningful way in terms of system origin or system function » (Huggett, 2007). They could be measured and mapped without connection between the processes and the forms. Figure 1 presents a schematic paraglacial landforms system.
Fig. 1 – Schematic paraglacial landforms system.
Fig. 1 – Représentation schématique des formes du système paraglaciaire.
4Secondly, process systems, which are also called cascading or flow systems, are defined by A. Strahler (1980) as « interconnected pathways of transport of energy and matter or both, together with such storages of energy and matter as may be required ». C. Ballantyne (2002) proposed a simplified paraglacial sediment cascade showing the principal primary and secondary sediment stores and main sediment transfer processes (fig. 2).
Fig. 2 – Simplified paraglacial sediment 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).
5Thirdly, process-form systems, also styled process-response systems, are defined as « an energy-flow system linked to a form system in such a way that system processes may alter the system form and, in turn, the changed system form alters the system processes » (Huggett, 2007). The systems approach allows us to study the sub-systems of the paraglacial landsystem in an integrated way and to focus on the fluxes of energy and sediments as a response to climate change.
6Following the definition of paraglacial by C. Ballantyne (2002), it is suggested to use the concept of paraperiglacial to define: « earth-surface processes, sediment accumulations, landforms, landsystems and landscapes that are directly conditioned by permafrost thaw-degradation ». This paraperiglacial concept could be used in all periglacial environments affected by climate change at both local and global scales. Processes like meltwater flow, fluvial reworking, runoff, debris flow, thermokarst thaw and collapse can be assimilated into paraperiglacial processes, and several forms of mass movements, thermokasrt features and deposits can be considered as paraperiglacial sediment stores (fig. 3).
Fig. 3 – Simplified paraperiglacial sediment cascade.
Fig. 3 – Schéma simplifié du transfert sédimentaire en cascade en milieu parapériglaciaire.
7During source-to-sink transport, paraperiglacial processes succeed to periglacial processes and preceed azonal processes. The “paraperiglacial” concept defines processes associated with permafrost thaw-degradation, a period over which paraperiglacial processes operate and areas in cold non-glacial environments affected by those changes.
8Russian Quaternary scientists were the first to use the term paraperiglacial to describe a period characterized by climate warming during interglacial stages (Velichko and Timireva, 1995), or to describe areas affected by interglacial variations in the vegetation cover resulting from climate change (Gribchenko and Kurenkova, 1997). They used this term to describe periglacial landsystems affected by warming and the consequent rapid degradation of permafrost, 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 settlement areas by Palaeolithic human groups (Gribchenko and Kurenkova, 1997). Previously, in another semantic way, A. Corte (1983, 1986) used the terms “paraperiglacial”, “parageocryogenic” and “paraglacial” to describe the spatial distribution of processes and landforms in the Central Andes. He used “paraglacial” to describe all facies of covered ice. The term “para-periglacial” or “para-geocryogenic” was used to define the seasonal ground-freezing region. This term was defined in a letter of 20 September 1982 entitled “the paraperiglacial environment” by Kowalkowski as follows: “a temperate climate zone with periodic freezing and thawing of soils and waste, seasonal snow and underground ice”.In the coastal sub-system, the term para-periglacial is used by R. Blanco-Chao et al. (2007), following the definition of paraglacial coasts by D. Forbes and J. Syvitski (1994) to describe Holocene conditions on shore platforms in Galicia and the former and continuing influence of periglacial and fluvio-nival deposits on coastal evolution and dynamics. They proposed a model to describe the recent evolution of what they called “a para-periglacial system” (Blanco-Chao et al., 2007). In para-periglacial systems, the relationship between intertidal rock strengh and tidal level may be a function of the occurrence, and nature of, continental slope deposits that impinged on the coastal domain. With rising Holocene sea level, erosion and retreat of sedimentary cliff deposits caused abrasion zones to migrate landwards, as the coarse sediments were released from the deposits; the effect of this migration may be preserved today in the form of a positive relationship between rock hardness and intertidal elevation. In this context, most coasts on both sides the Atlantic Ocean underwent a paraperiglacial evolution during the Holocene and during other interglacial eras, such as the Eemian. For instance, all French coasts covered by continental deposits during the Weichselian period could be considered to be para-periglacial coasts (fig. 4).
Fig. 4 – Paraperiglacial coast in Normandy, Ecalgrain bay.
Fig. 4 – Côte parapériglaciaire en Normandie, Baie d’Ecalgrain.
A: basement with Cambrian sandstones; B: cliffs cut in the continental periglacial deposits (head); C: Pleistocene solifluction deposits that constitute the main stock of available sediments for the beach of the foreground (photo: D. Mercier, 2002).
A : socle constitué de grès cambriens ; B : falaises taillées dans les dépôts périglaciaires continentaux (head) ; C : dépôts de solifluxion du Pleistocène formant l’essentiel du stock de sédiments disponibles pour la plage du premier plan (photo : D. Mercier, 2002).
9Those 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 deposits called “head” are reinterpreted as evidence of paraglacial conditions because those deposits represent meltout tills that were reworked by solifluction and mudflow, and were active during a period of immediate postglacial readjustment (Wright, 1991; Ballantyne and Harris, 1994). For D. Brunsden (2001) the paraglacial and para-periglacial supply of sediment to many beaches became depleted due to sea-level rise in the late Holocene. In mountainous areas, G. Ibañez-Palacios and A. Ahumada (2006) used the term “paraperiglacial” as a synonym of “parageocryogenic” for a zone, from 2 000 m to 4 000 m a.s.l., on the Eastern slope of Aconquija Range, characterized by seasonal ground freezing below the lowest termini of rock glaciers. C. Le Cœur (2007) described post Little Ice Age rock glacier evolution in the Alps, in the Cerces massif, and used the term paraperiglacial to characterize a dynamic associated with runoff, which reworks parts of rock glacier deposits and contributes to subsequent landform modification. The term “paraperiglacial” is proposed by Le Coeur to indicate also the sequence of evolution of a landform built by periglacial dynamics and non-frost specific processes, which henceforth affect the surface of the landform. This dynamic cannot evacuate the totality of the accumulated sediment in cold conditions, but it reshapes it and can contribute to its fossilization under slope deposits.
10By definition, the paraglacial and paraperiglacial landsystems are active over a temporal sequence, which corresponds to a morphogenic readjustement. Traditionally, it fits in time between the cold period and the period characterized by “normal” or common slope to river erosion. The paraglacial phase thus possesses a limited, calculable life expectancy from the quantification of the deposited volumes, the rates of denudation and the dating of the beginning of the phase. It is characterized by a curve of sediment supply corresponding to a negative exponential function (Ballantyne, 2003). The life time of the paraglacial and paraperiglacial sequence depends on the amount of sediment stock to be reshaped; on the rate of the processes and also on the climatic parameters and the geographical location of the catchment; on the postglacial vegetation cover; on the size, on the orographic and on the geologic nature of catchment, in which the paraglacial and paraperiglacial processes operate. After exhaustion of the sedimentary stocks, the paraglacial and the paraperiglacial sequences end. The paraglacial and paraperiglacial landsystems are likely to react to climate change because they rely on an ice-cold water stock (glacier and permafrost) generated during a previous glaciation. Climate change will affect the external inputs (thermal flow of energy; water and solids inflow) and affects the landystems through distinct endogenous variables as the rate and the nature of the processes or the vegetation cover. Climate change acts upon the landystems at various temporal scales. During interglacial periods (Eemian, Holocene), the paraglacial and paraperiglacial periods of tens of thousand years. During the post-Little Ice Age period, the sequence is of the order of the century and is superimposed on the Holocene, while the LIA constituted a period of slowing down of the paraglacial Holocene dynamics. At a consequence of recent abrupt climate changes, paraglacial and paraperiglacial activity are visible at the ten-year scale and are superimposed on the century scale. A period of acceleration of global warming (1980-2007) followed a period of cooling (1940-1970). Despite differences in intensity of the changes and their duration, important reactions of the paraglacial and paraperiglacial landsystems can be determined.
11The response of the systems to forcing takes place over a range of time spans ranging from an immediate reaction to several millennia. Several examples are as follows. Firstly, immediate response could be analysed for fluvial sub-system. Jökulhlaups are crises of strong intensity, which correspond typically to a forcing of sub-glacial volcanic origin and are often observed in Iceland (Kristmannsdóttir et al, 1999; Russell and Knudsen, 1999; Knudsen et al, 2001; Stefansdóttir and Gislason, 2005). Jökulhlaupscause sandur aggradation, which may be eroded within few years (Smith et al, 2006; Roussel, 2008). These floods can also be the result of break of the ice dam (Björnsson, 1992; Étienne et al., 2008). On the other hand, some debris flows in mountain environment with glacier retreat are associated with glacial lake outbursts and usually occur during the summer months when they coincide with intense glacier ablation and meltwater production (Passmore et al., 2008). Debris flows in this high altitude environment, with the availability of unconsolidated sediments, had been considered as an important component of the paraglacial response to glacier retreat since the end of the Little Ice Age. Secondly, glacier retreat leaves moraines, quickly reshaped by glidings and poured by fragments activated by streaming of cast iron of the dead ice within the deposit. This dynamic is paraglacial by nature and ends within decades following the disappearance of dead-ice (Curry, 1999; Curry and Ballantyne, 1999; Mercier, 1997, 2001). For paraperiglacial landsystem, thermokarst landscapes or retrogressive thaw-slump on coastal zone (Hill et al., 1994; Lantuit and Pollard, 2008) responds to climate warming on a same decadal scale. Thirdly, the most prominent modification of the relief at one hundred years scale is probably associated with fluvial sub-systems. Progradation of seaward downstream to the conquering river systems were quantified in Svalbard (Mercier and Laffly, 2005; Roussel, 2005). A predictive model built from data from forty-six rivers of the Arctic showed that for a 2 °C rise rivers would increase their sedimentary discharge by 22% (Syvitski, 2002). B. Peterson et al. (2002) studied the six largest Eurasian arctic rivers (including the Lena) and calculated an increase in annual average rate of discharge of 2.0 ± 0.7 km3, which corresponds to an approximate 7% increase. Recent climate warming induces the propagation of a thawing line within the frozen riverbank of the Lena River and this paraperiglacial process produces niches, which contribute to the disequilibrium of the bank by thermokarstic subsidenceand subsequent large slumps along the river-banks (Walker, 1983; Costard et al., 2007). Since the end of the Little Ice Age, palsa degradation in subartic is influenced by several regional and local factors including increase air temperature, changes in depth of snow cover, water level fluctuations in the river (Vallée and Payette, 2007). Fourthly, one of the consequences of the deglaciation is the exposure of formerly englaciated mountain walls. The disappearance of the ice induces the occurrence of major glidings, collapses or massive collapses (0.25-3.0 km2), often representing lethal threats to infrastructure and human settlements. They can also introduce slow progressive deformations of hillside, either still the periodic adjustments of hillsides by falls or glide of small sizes (André, 1997; Ballantyne, 2008; Cossart et al., 2008; Iturrizaga, 2008; Jarman, 2006; Sellier, 2008). This paraglacial dynamics can occur within a few millennia after deglaciation as evidenced by cosmonucleous datings (Ballantyne et al., 2008). Finally, the analysis of offshore sedimentation in the marine sub-system provides proxies for the reconstitution of glacial and paraglacial sequences and rates of sedimentation over the last three million years (Ottesen et al., 2005a and 2005b). At least 1000 m of sediments accumulated on the Scandinavian margin during this period of time (Rise et al., 2005).
12The spatial limits of the system are inherently dependant on the time scale at which the system is considered. Commonly glacial and interglacial periods are known to be peculiar features of the Pleistocene. According to several authors, the maximum ice-sheet extent took place during this period (fig. 5) (Andersen and Borns Jr., 1997; Grosswald, 1998; Ehlers and Gibbard, 2004a and 2004b; Svendsen et al., 2004).
Fig. 5 – Spatial distribution of palaeo- and active paraglacial and paraperiglacial landsystems and landscapes.
Fig. 5 – Répartition spatiale des systèmes et des paysages paraglaciaires et parapériglaciaires hérités et actifs.
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 permafrost extent during Pleistocene period.
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.
13During the Pleistocene, glaciations widely affected the high latitudes and the high altitudes of the Earth and were followed by inherited paraglacial sequences. The Holocene contains an active paraglacial period, which follows the last cold Pleistocene period. The extent of the ice-sheets during the cold periods of the Pleistocene was characterized by paraglacial activity during interglacial periods. The great glaciations and post-glacial sequences were times of global reorganization in the Earth’s hydrological system. Glaciations and paraglacial sequences caused profound changes in all elements of the system (glaciers, rivers, lakes, oceans, atmospheric moisture). On the continental areas, the main part of North America, Scandinavia, and northern Russia were covered by ice-sheets and became areas of paraglacial activity during the Holocene deglaciation. In high altitudes areas (Cantabrique, Pyrenées, Alps, Caucasus, Pamir, Tien Shan, TransBaikalian mountains…), mountainous areas and piemonts were completely ice-covered or semi-continuously covered by ice caps and extensive glacier complexes. Paraglacial sequences also affected coastal areas (e.g., south Baltic coast) and continental margins with offshore deposits (e.g., North sea, Scandinavian continental shelf, Barents sea, Kara sea, Laptev sea). All of these areas correspond nowadays to landscapes of inherited paraglacial deposits. Contemporary global warming affects in particular glacier margins that are known to have readvanced during the Little Ice Age. All glacial forelands in Arctic and alpine areas are experiencing paraglacial processes under present warming. The expected global warming for the twenty-first century will result in significant impacts on present glacial areas in mountains and could result in the appearance of new areas for paraglacial dynamics.
14In the vicinity of glaciers and in non-glacial cold environments, permafrost landscapes underwent a similar geomorphological readjustment that can be considered as paraperiglacial. One quarter of the earth’s land surface currently experiences periglacial conditions. During colder periods of the Pleistocene, an additional one fifth of continental areas were affected by periglacial conditions associated with the presence of continuous permafrost around ice-sheets in North America, Europe and Russia (Van Vliet-Lanoë and Lysitsyna, 2001; French, 2007). At the time scale of the Quaternary, periglacial environments roughly coincided with various advances and retreats of continental ice sheets. During the Holocene period and the retreat of North American, Scandinavian and Russian ice sheets, periglacial conditions became predominant. With the warming of high latitude environments, periglacial areas also experience paraperiglacial period, which could be superimposed on inherited paraglacial landscapes. An example could be retrogressive thaw slump activity on Herschel Island in the Canadian Arctic, which is due to paraperiglacial processes (warming permafrost, increasing active layer depths and thermokasrt activity) and environmental changes (sea level rise, reduction in sea ice extent and duration, increasing storms impacts) affecting inherited paraglacial deposits (Lantuit and Pollard, 2008).
15The paraglacial and paraperiglacial landsystems are complex open cascading systems very sensitive to climate change. The responses of the paraglacial landsystem to climate change span over time scales ranging from the immediate reaction to the million years. The life expectation of the paraglacial landsystem is certainly limited, but can be prolonged for several centuries or millenniums in areas of sediment storage (Cossart and Fort, 2008). A paraglacial period follows a glacial period and would logically be followed by a period of “normal” erosion. Due to the brief duration of interglacial periods however, the paraglacial phases cannot be finished totally before the beginning of the following glaciation. Climate change leads to paraglacial and paraperiglacial geomorphological adjustment. Areas unglaciated during the Pleistocene underwent a glacial period, then a paraglacial crisis, followed by a periglacial phase now affected by a paraperiglacial crisis. The concepts of paraglacial and paraperiglacial periods provide a useful framework for reconstruction of glaciated and periglacial landscapes evolution.