1Iceland is an ideal setting to conduct geosciences research. Certainly, the inherent internal geodynamics of this volcanic island, connected to a hot spot and located on the Mid-Atlantic Ridge, contribute to its attractiveness for many geological researches, including geochemistry and tectonics. However, external geodynamics and, more specifically, methodological contributions and analyses of low-frequency but high-energy events, are emphasized in this special issue of the journal Géomorphologie: relief, processus, environnement dedicated for the first time to Iceland. Since its launching in 1995 the journal has published only three papers on researches in Iceland (Decaulne, 2002; Etienne and André, 2003; Roussel et al., 2008).
2Spatially, the six studies included in this issue are divided into four contrasting Icelandic regions (fig. 1). Julien Coquin, Denis Mercier, Olivier Bourgeois, Thierry Feuillet and Armelle Decaulne study gravitational spreading and its impact on the Stífluhólar landslide in the Tröllaskagi massif, east of Skagafjörður; Aurore Peras, Armelle Decaulne, Etienne Cossart, Julien Coquin and Denis Mercier analyze post-glacial landsliding at a regional level for the Westfjords peninsula. In the paper of Erwan Roussel, Jean-Pierre Toumazet, Philip Marren, Etienne Cossart and the one of Philip Marren, the vast sandur areas in south-eastern Iceland are studied, south of Vatnajökull. Ronan Autret, Serge Suanez, Bernard Fichaut and Samuel Etienne selected the rocky coasts of the Reykjanes peninsula (south-west Iceland) to study storm deposits. Finally, the synthesis proposed by Armelle Decaulne covers the whole island, extracting information obtained during the last four decades by over thirty researchers using lichenometry in different sites.
Fig. 1 – Location of study sites investigated in this special issue.
1: North Tröllaskagi in Coquin et al.; 2: Westfjords in Peras et al.; 3: South-east Vatnajökull in Roussel et al.; 4: Skeiðará in Marren; 5: South Reykjanes peninsula in Autret et al.; 6: lichenometry sites in Decaulne.
3Thematically, five of the six papers that are gathered in this volume address three aspects of the sedimentary cascade from slope dynamics, with the fundamental issue of the gravitational spreading of mountain ranges and its impact on mass movements (Coquin et al.), post-glacial landslides (Peras et al.), river (Marren, Roussel et al.) and coastal dynamics (Autret et al.). This special issue also addresses a methodological issue on a subject that became controversial in the last years: lichenometry and its limits as a chronological marker (Decaulne).
4Paraglacial geomorphology is a major theme in this issue: the study of deglaciation impacts in Iceland is the core of the contributions from Coquin et al., Peras et al., Marren, and Roussel et al. Besides, low-frequency but high-energy events are analyzed. Indeed, whether it deals with landslides (Coquin et al. and Peras et al.), outbursts of iceberg jams (Roussel et al.), jökulhlaup effects on sandar (Marren) or block deposits over cliff tops on the coasts (Autret et al.), all these dynamics and resulting landforms reflect the importance of these rare events in the morphogenesis.
5The paper by Julien Coquin, Denis Mercier, Olivier Bourgeois, Thierry Feuillet and Armelle Decaulne deals with an issue little addressed by the international community: the relationship between gravitational spreading and mass movements. They demonstrate the links between the deglaciation of the Holtshyrna ridge, its gravitational spreading and its role in the preparation of subsequent mass movements. These findings are based on field observations and on a large-scale geomorphological mapping of the deformations. The paper also offers dating elements for the Stífluhólar rock-slope failure, based on the use of tephrochronology and on an age-depth model, thus contributing to the establishment of a post-glacial chronology during the late Weichselian for these paraglacial dynamic in northern Iceland, consistent with results obtained in environments that have also encountered deglaciation like Scotland (Ballantyne et al., 2014) or the Alps (Crosta et al., 2013).
6The spatial analysis of rock-slope failures is conducted by Aurore Peras, Armelle Decaulne, Etienne Cossart, Julien Coquin and Denis Mercier at a regional scale in the Icelandic Westfjords. Systematic photo-interpretation, coupled with pattern recognition in the field, enables the authors to identify 186 deposits from rock-slope failures. Their spatial distribution shows a concentration near the coast, with the highest densities in the extreme NW (between Ísafjarðardjúp and Arnarfjörður), in a part of the Northeast (Strandir) and in the South (by the isthmus), and also in the fjords of northern Breiðafjörður. Using this new database, the spatial analysis shows that while topographic data (slopes) and structural (contact between basaltic and intrusive rocks, dip) play an important but expected role in the location of landslides, it is especially the chronology of deglaciation of this part of Iceland that controls the spatial distribution of rock-slope failures. Thus, the authors suggest that 73% of rock-slope failures may have occurred since the deglaciation period, between Bølling-Allerød and Younger Dryas, confirming the role of the paraglacial sequence for rock-slope failure initiation at this spatial and temporal scale.
7The paper by Erwan Roussel, Jean-Pierre Toumazet, Philip Marren and Etienne Cossart describes the formation of iceberg jams in the Öræfi region, south of the Vatnajökull ice cap, from photo-interpretation and field data from eleven glacier tongues. This process, although little known, plays a considerable role for sediment transfers in the forefield during deglaciation stages. This work is also an opportunity to test the hypothesis that iceberg jam floods corresponds to a self-organized critical system (Self-Organized Criticality). Using a simple digital model, the authors show that iceberg jam floods become less frequent and of similar intensity over time; the overall trend is related to the gradual calibration of the channel geometry at the outlet of the proglacial lake. They mainly show that self-organized criticality affecting iceberg jam floods reinforces the sporadic marginal sediment transfer and the morphological evolution of proglacial river systems.
8The paper by Philip Marren analyzes the morphology of bars within the Skeidará river and their evolution after the 1996 major jökulhlaup. The morphology of these deposits is controlled by the geometry of the floodway, in agreement with the "bar scaling theory" that shows that jökulhlaup deposits record a partial reworking thanks to the lateral mobility of braided rivers. Thus, reworked jökulhlaup bars have a higher length/width ratio than those from the primary jökulhlaup and generally retain the surface topography caused by the flood that had the greater geomorphic impact. This morphometric analysis identifies deposits from low frequency and high intensity floods.
9The field work of Ronan Autret, Serge Suanez, Bernard Fichaut and Samuel Etienne deals with cliff top storm deposits in the southern Reykjanes peninsula and offers a precise typology of megablocks from dynamic criteria regarding the removal of rocky material, transfer and deposition of these blocks.
10In her paper, Armelle Decaulne proposes an analytic and synthetic study of over thirty-five publications that used lichenometry in Iceland as a chronological marker. This contribution confirms, at the scale of Iceland, the criticisms published in the recent paper by Osborn et al. (2015) on lichenometry that has been widely used since the 1950s, especially in cold high latitude environments. For Armelle Decaulne, Iceland is not an ideal place to use this method as an absolute dating tool due to rapid environmental changes over short distances that affect the growth of lichens and lack of reference surfaces of old age; also, numerical dates proposed by several authors are highly dubious as very few can identify correctly the species, offering at the end chronological discrepancies with tephrochronology. If methodological cautions are most often taken (out of lichen identification, i.e. the raw data), the accuracy of the dating obtained by lichenometry is relatively reliable for the first decades of surface exposure. However, this article demonstrates that it is premature to use lichenometry as an absolute dating method, despite conclusions many studies pretend to reach in the recent decades.
Fig. 2 – Views of the different study sites/topics of this special issue.
a- the early Holocene Stífluhólar landslide dams the valley in Northern Iceland (photo A. Decaulne, July 2010); b- two contiguous landslides in the Icelandic Westfjords (photo A. Decaulne, September 2015); c- iceberg accumulation at the outlet of the glacial lake Jökulsarlón, south Vatnajökull (photo D. Mercier, August 2010); d- Skeiðarársandur and the braided river Skeiðará, south Vatnajökull (photo Þ. Sæmundsson, August 2009); e- view of cliffs of the southern coast of the Reykjanes peninsula, southwest Iceland (photo D. Mercier, July 2013); lichens on a boulder deposited by an avalanche in the Fnjóskadalur valley, Northern Iceland (photo A. Decaulne, August 2008).
11This special issue demonstrates the interest of geomorphologists to study for Nordic environments. It illustrates the dynamism of French research, especially of the younger generation, within the international community oriented towards understanding the evolution of polar and sub-polar environments, and Iceland in particular. It demonstrates the fundamental importance of low-frequency but high-magnitude events in the evolution of landforms, especially in cold environments during the deglaciation stage.