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Paraglacial geomorphology: Conceptual and methodological revival

Denis Mercier
p. 219-222

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1The geomorphological approaches to study the cold-environment evolution have proved consistently interesting from the beginning of the 19th century. Yet, the topic of global warming and consequences on thermically sensitive environments has revived multidisciplinary approaches in polar, subpolar and high-mountain environments. So, glacial regions and their margins arouse a renewed scientific interest, where geomorphology contributes to the revival of concept and methodology.

2Thus a paraglacial geomorphology has emerged at the international and national levels (Ballantyne, 2002; Mercier and Étienne, 2008; Mercier, 2008a). Presently geomorphologists find increasing interest in glacial environments (Benn and Evans, 1998; Evans, 2005; Stroeven and Swift, 2008), which for a moment were mainly studied by glaciologists and sedimentologists. Other geomorphological studies focus on periglacial regions. In the second part of the 20th century, periglacial geomorphology has overestimated mechanical weathering by freeze-thaw cycles and underestimated lithological parameters, duration and efficiency of cold climatic conditions (French and Thorn, 2006). Even today, the geomorphology of cold environments uses to be confined in a dichotomous approach between glacial and periglacial processes, illustrated for example by the choice of papers presented during the sixth International Conference on Geomorphology in Saragossa in 2005 (Vilaplana and Gray, 2007).

3Because the present climate change affects especially polar, subpolar and high-mountain environments, the geomorphologists raise several questions: what are the geomorphological consequences? What are the most active processes and frequent landform changes? At what rate, do the glacial regions and margins answer the thermal oscillations?

4So a new approach of these environments appears with the paraglacial concept (Mercier, 2007). Used since the 1960s (Godard, 1965), the term "paraglacial" was conceptualized by M. Church and J. Ryder in 1972. Previously, J. Ryder (1970; 1971a, b) recognised “paraglacial alluvial fans” in a geomorphological study in the South-central British Columbia. She concluded that their construction was controlled by geomorphic processes working during the transition from glacial to post-glacial conditions, when abundant glacial debris were temporarly present. She also demonstrated that these paraglacial fans were temporary features. M. Church and J. Ryder (1972) gave two meanings to the “paraglacial” concept. The first one is used to define non-glacial processes that are directly conditioned by glaciations, peculiarly proglacial processes and those occurring around and within the margins of a former glacier, which result directly from the earlier presence of ice. M. Church and J. Ryder described mud-flow, debris flow, fluvial aggradation and fluvial reworking as paraglacial processes. The second one has a temporal meaning: a period is defined as “paraglacial” when paraglacial processes occur. They proposed a schematic diagram of the pattern of sedimentation during a paraglacial period over which the rate of sediment yield follows an asymptotic decline toward the geological “norm”. D.I. Benn and D.J.A Evans (1998) claimed that the term “paraglacial period” is more appropriate than “paraglacial environment”, because processes are not specific to such environments. The paraglacial period is characterized by high rates of sediment delivery, either in slope, fluvial or aeolian systems, and it could be triggered by the instability of unconsolidated glacigenic sediments and oversteepened rock slopes (Ravanel and Deline, this issue). The paraglacial period theorically ends once sediment yield drops to rates typical of unglaciated catchments. M. Church and O. Slaymaker (1989) claimed that the paraglacial period is not restricted to the closing phases of glaciation but may extend into the ensuing nonglacial interval. According to the paraglacial concept, recently deglaciated areas are often in an initial unstable or metastable state and yet vulnerable to rapid modification by subaerial agents. The paraglacial period is the period of readjustment from glacial to non-glacial conditions. Recently, C.K. Ballantyne (2002) reviewed paraglacial geomorphological researches and proposed a global definition. For him, paraglacial geomorphology studies earth-surface processes, sediments, landforms systems and landscapes that are directly conditioned by former glaciation and deglaciation. He identified six paraglacial landsystems: rock slopes, drift-mantled slopes, glacier forelands, and alluvial, lacustrine and coastal systems.

5In fact, the paraglacial concept has several meanings and concerns period, processes, landforms, environments and landsystems. It leads to a true “paraglacial geomorphology” for the same reasons as periglacial, glacial or coastal geomorphology.

6This conceptual revival came along with a methodological revival. The system approach is peculiarly appropriate to study paraglacial processes and associated landforms, in grasping inputs, thermal forcing and energy fluxes. It quantifies the consequences on sediment transport, reserve, trap and shifting. Finally, it estimates the outputs of the system (Mercier, Roussel et al., this issue). The system approach implies feedback analysis (Cossart, this issue). Paraglacial sequences need precise dating to be defined. For example, postglacial rock-slope failures are dated by cosmogenic analysis (Ballantyne, 2008; Cossart et al., 2008). For the more recent evolution, following the end of the Little Ice Age, lichenometry, which has been used since the 1950s, gives useful data. The statistical analysis, based on extreme values and Bayesian theories, gives a new method of indirect dating (Roussel et al., this issue.). The quantification of sediment transport, deposit and spatiotemporal shifting was improved by laser scanning (Ravanel and Deline, this issue) and geographical information systems (GIS). Thanks to remote sensing and GIS it is possible to map sediment transport from sources to sinks (Cossart, Roussel et al., this issue).

7The paraglacial geomorphology and system approach have considerably improved the knowledge of current and past evolution of recently deglaciated areas. Although major gaps still remain, the paraglacial concept offers a general hypothesis to better understand the Holocene environmental change after the disappearance of glacier ice.

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Bibliographie

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Denis Mercier, « Paraglacial geomorphology: Conceptual and methodological revival », Géomorphologie : relief, processus, environnement, vol. 14 - n° 4 | 2008, 219-222.

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Denis Mercier, « Paraglacial geomorphology: Conceptual and methodological revival », Géomorphologie : relief, processus, environnement [En ligne], vol. 14 - n° 4 | 2008, mis en ligne le 15 avril 2009, consulté le 24 septembre 2017. URL : http://geomorphologie.revues.org/7395

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Denis Mercier

Université de Nantes, laboratoire Géolittomer, CNRS - UMR 6554 LETG, campus du Tertre, BP 81227, 44312 Nantes cedex 3. denis.mercier@univ-nantes.fr

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