Un inventaire des aérophotographies du massif du Mont Blanc

National audienceLe massif du Mont Blanc, espace frontalier entre trois pays alpins, constitue un laboratoire scientifique de premier ordre pour étudier plusieurs des problématiques de recherche du laboratoire EDYTEM, notamment en ce qui concerne les domaines glaciaires et périglaciaires. Dans ce contexte, un inventaire des aérophotographies couvrant le massif et disséminées au sein de plusieurs organismes a été effectué. Près de 5000 photogrammes pris au cours de 92 missions ont ainsi été identifiés, assurant une couverture homogène et à haute résolution temporelle du massif depuis la fin des années 1930. Cet inventaire a donné lieu à une première réflexion sur la constitution d'une base de données de ces documents pour faciliter les recherches documentaires futures

Thermal and dynamic behaviour of supraglacial clasts and the origin of sorting in supraglacial debris covers

The transition zone from a discontinuous to a continuous debris cover is an extensive part of many glacier ablation zones. Although responsible for the highest specific melt rates of debris-covered glaciers, transition zones have received little research and are poorly understood. Here we consider the interactions between emergent clasts and melting ice surfaces at Glacier d'Estelette and Miage Glacier (Italian Alps). Debris-ice interactions are complex because dispersed heterogenous debris both enhances and retards melt rate in the same locality, depending on the distribution of clast sizes. Observations reveal that thermal and dynamic clast interactions with the glacier surface increase the transport rate of coarse clasts, and initiate vertical sorting at the point when a continuous debris layer forms. This happens because, in summer, clasts exceeding the critical thickness for melt slide over the glacier surface. In contrast finer thermally-embedded material is transported at ice surface velocity and become covered by coarser material from upslope. Once established, debris-cover texture allows sorting to develop as the cover thickens downglacier. A two-layer temperature profile results, in which a coarse, drier clast layer of low thermal conductivity overlies a finer-grained, moist layer of higher thermal conductivity. Transition-zone processes establish inverse grading at the initiation of a debris cover, allowing subsequent sorting to operate as the cover thickens downstream. The processes by which this occurs are unknown, but analogy with periglacial active layers suggests convection within a frost-susceptible lower fine layer and eluviation of fines supplied by aeolian deposition and in-situ clast distintegratio

Neige et Glace de Montagne : Reconstitution, dynamique, pratiques

Collection edytem n°8International audienceLes cahiers (de géographie, géologie et paléoenvironnement) de la Collection EDYTEM visent à faire connaître les recherches menées au sein du laboratoire EDYTEM ou de programmes dans lequel nous sommes impliqués. Si la priorité est la diffusion de nos travaux dans les revues majeures de nos communautés et les congrès internationaux, il nous paraît également important de mettre en avant la transversalité des recherches du laboratoire qui mobilise plusieurs champs disciplinaires autour de mêmes objets d'étude. Il est, en effet, difficile de mettre en avant cette dimension par le seul biais des publications dans des revues ou des communications qui répondent avant tout à des logiques disciplinaires. La Collection EDYTEM a cette mission : rendre visible la démarche interdisciplinaire du laboratoire autour d'entrées fédératrices : les systèmes hydrothermaux de montagne, les systèmes nivo-glaciaires et les systèmes karstiques. Trois entrées qui caractérisent les objets privilégiés d'étude du laboratoire. Après le numéro dédié à l'Aven d'Orgnac porté par " l'axe karst ", ce nouveau numéro illustre les recherches menées sur la haute montagne et les systèmes nivo-glaciaires. Le prochain numéro également prévu en 2009 traitera de l'hydrothermalisme de montagne. Soulignons que ces trois axes ont été mis en place il y a un peu plus de deux ans. Le fait que chacun d'entre eux produise en peu de temps des synthèses originales, reflète la rapide mobilisation des différents champs disciplinaires du laboratoire sur ces objets de recherche. Ce numéro " Neige et glace " illustre parfaitement cette dynamique. La participation très active des doctorants, comme contributeurs mais aussi animateurs des réunions de travail et de chantiers de recherche en est un bon exemple. Tout comme la structuration du numéro, qui aborde les reconstitutions paléoenvironnementales (de la dernière grande glaciation au Petit Âge Glaciaire), les dynamiques actuelles, à la fois reflets et indicateurs du changement climatique, et les incidences de ce changement sur le tourisme hivernal, les risques en montagne. Ces trois entrées illustrent les apports respectifs des trois équipes du laboratoire. Les 21 contributions originales de ce cahier reflètent la dynamique du laboratoire sur la montagne et les priorités mises à la fois sur les outils (laserscanning, métrologie, dendrochronologie, photogrammétrie, imagerie, modélisation...), les recherches doctorales (T. Barth, N. Cayla, S. Coutterand, P. Paccard, L. Ravanel et M. Le Roy) et l'accueil de chercheurs et de post-doct étrangers (S. Gruber, M. Kirkbride). Ce numéro a le mérite de poser clairement les recherches menées par le laboratoire sur la haute montagne, la neige et les glaciers et sa contribution dans les sciences de l'environnement

La dendroglaciologie ou l'apport des l'étude des cernes d'arbres pour la reconstitution des fluctuations glacières holocènes

La dendroglaciologie est l'un des champs de la dendrochronologie qui s'applique à la reconstitution des fluctuations glaciaires en analysant le signal climatique et/ou géomorphologique contenu dans les cernes de croissance des arbres. Son utilisation est deve­nue courante aujourd'hui pour dater les récurrences glaciaires dans les régions où les glaciers ont pénétré des secteurs boisés lors des péjorations climatiques de l'Holocène. L'intérêt majeur de cette méthode est de permettre une meilleure résolution (annuelle dans des conditions favorables) que les autres méthodes de datation utilisées sur la période holocène. Nous présentons ici une revue des différentes méthodes utilisées en dendroglaciologie à partir d'exemples pris dans la littérature ; une attention particulière est portée aux travaux effectués dans la région alpine

Estimating surface water availability in high mountain rock slopes using a numerical energy balance model

Water takes part in most physical processes that shape the mountainous periglacial landscapes and initiation of mass wasting. An observed increase in rockfall activity in several mountainous regions was previously linked to permafrost degradation in high mountains, and water that infiltrates into rock fractures is one of the likely drivers of these processes. However, there is very little knowledge on the quantity and timing of water availability for infiltration in steep rock slopes. This knowledge gap originates from the complex meteorological, hydrological and thermal processes that control snowmelt, and also the challenging access and data acquisition in the extreme alpine environments. Here we use field measurement and numerical modeling to simulate the energy balance and hydrological fluxes in a steep high elevation permafrost affected rock slope at Aiguille du Midi (3842 m a.s.l), in the Mont-Blanc massif. Our results provide new information about water balance at the surface of steep rock slopes. Model results suggest that only ~25 % of the snowfall accumulates in our study site, the remaining ~75 % are redistributed by wind and gravity. Snow accumulation depth is inversely correlated with surface slopes between 40&deg; to 70&deg;. Snowmelt occurs between spring and late summer and most of it does not reach the rock surface due to the formation of an impermeable ice layer at the base of the snowpack. The annual effective snowmelt, that is available for infiltration, is highly variable and ranges over a factor of six with values between 0.05&ndash;0.28 m in the years 1959&ndash;2021. The onset of the effective snowmelt occurs between May and August, and ends before October. It precedes the first rainfall by one month on average. Sublimation is the main process of snowpack mass loss in our study site. Model simulations at varying elevations show that effective snowmelt is the main source of water for infiltration above 3600 m a.s.l.; below, direct rainfall is the dominant source. The change from snowmelt-dominated to rainfall-dominated water availability is nonlinear and characterized by a rapid increase in water availability for infiltration. We suggest that this elevation of water availability transition is highly sensitive to climate change, if snowmelt-dominated permafrost-affected slopes experience an abrupt increase in water input that can initiate rock slope failure.</p

Processes at the margins of supraglacial debris cover: quantifying dirty ice ablation and debris redistribution

Current glacier ablation models have difficulty simulating the high‐melt transition zone between clean and debris‐covered ice. In this zone, thin debris cover is thought to increase ablation compared to clean ice, but often this cover is patchy rather than continuous. There is a need to understand ablation and debris dynamics in this transition zone to improve the accuracy of ablation models and the predictions of future debris cover extent. To quantify the ablation of partially debris‐covered ice (or ‘dirty ice’), a high‐resolution, spatially continuous ablation map was created from repeat unmanned aerial systems surveys, corrected for glacier flow in a novel way using on‐glacier ablation stakes. Surprisingly, ablation is similar (range ~5 mm w.e. per day) across a wide range of percentage debris covers (~30–80%) due to the opposing effects of a positive correlation between percentage debris cover and clast size, countered by a negative correlation with albedo. Once debris cover becomes continuous, ablation is significantly reduced (by 61.6% compared to a partial debris cover), and there is some evidence that the cleanest ice (<~15% debris cover) has a lower ablation than dirty ice (by 3.7%). High‐resolution feature tracking of clast movement revealed a strong modal clast velocity where debris was continuous, indicating that debris moves by creep down moraine slopes, in turn promoting debris cover growth at the slope toe. However, not all slope margins gain debris due to the removal of clasts by supraglacial streams. Clast velocities in the dirty ice area were twice as fast as clasts within the continuously debris‐covered area, as clasts moved by sliding off their boulder tables. These new quantitative insights into the interplay between debris cover characteristics and ablation can be used to improve the treatment of dirty ice in ablation models, in turn improving estimates of glacial meltwater production

Paraglacial Rock Slope Adjustment Beneath a High Mountain Infrastructure—The Pilatte Hut Case Study (Écrins Mountain Range, France)

Landslides triggered by shrinking glaciers are an expected outcome of global climate change and they pose a significant threat to inhabitants and infrastructure in mountain valleys. In this study we document the rock slope movement that has affected the Pilatte hut (2,572 m a.s.l.) in the Écrins range (French alps) since the 1980s. We reconstructed the geometry of the unstable rock mass using Terrestrial Laser Scanning and quantified the unstable volume (~400,000 m3). Field observations and annual crack surveys have been used to identify the dynamics of past movements. These movements initiated in the late 1980s and have accelerated since 2000. The current trend seems to be toward a relative stabilization. Reconstruction of the glacier surface using past images taken since 1960 and “Structure from Motion” photogrammetry showed that the glacier probably applied stresses to the rock slope during its short-lived advance during the 1980s, followed by debuttressing caused by rapid surface lowering until the present day. The relationship between observed crack propagation and glacier surface change suggests that the rock slope instability is a paraglacial response to glacier surface changes, and highlights that such responses can occur within a decade of glacier change

Do cladistic and morphometric data capture common patterns of morphological disparity?

The distinctly non-random diversity of organismal form manifests itself in discrete clusters of taxa that share a common body plan. As a result, analyses of disparity require a scalable comparative framework. The difficulties of applying geometric morphometrics to disparity analyses of groups with vastly divergent body plans are overcome partly by the use of cladistic characters. Character-based disparity analyses have become increasingly popular, but it is not clear how they are affected by character coding strategies or revisions of primary homology statements. Indeed, whether cladistic and morphometric data capture similar patterns of morphological variation remains a moot point. To address this issue, we employ both cladistic and geometric morphometric data in an exploratory study of disparity focussing on caecilian amphibians. Our results show no impact on relative intertaxon distances when different coding strategies for cladistic characters were used or when revised concepts of homology were considered. In all instances, we found no statistically significant difference between pairwise Euclidean and Procrustes distances, although the strength of the correlation among distance matrices varied. This suggests that cladistic and geometric morphometric data appear to summarize morphological variation in comparable ways. Our results support the use of cladistic data for characterizing organismal disparity

Categorical versus geometric morphometric approaches to characterizing the evolution of morphological disparity in Osteostraci (Vertebrata, stem Gnathostomata)

Morphological variation (disparity) tends to be evaluated through two non-mutually exclusive approaches: (i) quantitatively, through geometric morphometrics, and (ii) in terms of discrete, ‘cladistic’, or categorical characters. Uncertainty over the comparability of these approaches diminishes the potential to obtain nomothetic insights into the evolution of morphological disparity, and the few benchmarking studies conducted so far show contrasting results. Here, we apply both approaches to characterising morphology in the stem-gnathostome vertebrate clade Osteostraci, in order to assess congruence between these alternative methods as well as to explore the evolutionary patterns of the group in terms of temporal disparity and the influence of phylogenetic relationships and habitat on morphospace occupation. Our results suggest that both approaches yield similar results in morphospace occupation and clustering, but also some differences indicating that these metrics may capture different aspects of morphology. Phylomorphospaces reveal important convergence towards a generalised ‘horseshoe’-shaped cranial morphology and two strong branching trends involving different major groups of osteostracans (benneviaspidids and thyestiids), which probably reflect adaptations to different lifestyles. Temporal patterns of disparity recorded by categorical and morphometric approaches differ considerably, capturing disparity maxima at very different times of the evolutionary history of the group. Disparity patterns recorded by the categorical approach parallel taxonomic diversity dynamics, likely reflecting a bias in facies representation rather than a real biological signal. This work provides evidence supporting that categorical and continuous data do not always capture morphological disparity in equivalent ways and that discrepancies reflect differences in the potential of each data type for characterizing more or less inclusive aspects of overall phenotype