Essai de quantification de la déflation en milieu subpolaire, (Skeiðarársandur, Islande).

National audienceThe Skeiðarársandur is a large glaciofluvial plain in the south of the Vatnajökull ice cap. In November 1996, a jökulhlaup triggered by a subglacial eruption and produced a catastrophic flood which incised the Skeiðarársandur. Those incised channels trap currently part of the wind transport (essentially the transport by saltation). Measures of the dominant orientations of the vegetation and petrographical analysis of the aeolian deposits provide us the opportunity to geographically delimit the blownout areas. Thus, with the ratio of (1) the loess deposit volume accumulated in the jökulhlaup channels since 1996 and (2) the blownout land surfaces, a minimum deflation rate for the east part of the Skeiðarársandur is calculated. According to our results, the Skeiðarársandur loses 0.05 to 0.1 mm in thickness due to aeolian transport by saltation each year

L’évolution morphologique récente du réseau hydrographique sur les marges des glaciers Lovén, presqu’île de Brøgger (Spitsberg, 79° N)

Les changements climatiques postérieurs au Petit Âge Glaciaire entraînent pour les milieux polaires la mise en place de processus paraglaciaires (Church et Ryder, 1972) où le ruissellement domine les processus zonaux (glaciaires et périglaciaires). En effet, au Spitsberg, la décrue enregistrée par les glaciers de la presqu’île de Brøgger provoque la libération d’eau liquide qui gagne le Kongsfjord en dessinant sur les marges proglaciaires un réseau hydrographique complexe et très mobile. Cependant, nous avons pu mettre en évidence que cette mobilité diminue au fil des années. Le réseau hydrographique proglaciaire adopte une organisation de plus en plus stable marquée par la diminution du nombre de drains et une dynamique de hiérarchisation malgré son extrême jeunesse.In polar areas the post Little Ice Age warming generates paraglacial processes (Church and Ryder, 1972) marked by the prevalence of run-off over the zonal (glacial and periglacial) processes. In Spitsbergen, the retreat of the Brøgger peninsula’s glaciers induces the release of large amounts of proglacial liquid water reaching the Kongsfjord. This results into a complexe and very mobile hydrographic network development on the proglacial margins. However, we have been able to demonstrate that this mobility decreases with time. The hydrographic network adopts a more and more stable organization marked by the reduction of the number of channels and a growing hierarchisation in spite of its extreme youth

Iceberg jam floods in Icelandic proglacial rivers: testing the self-organized criticality hypothesis

In this paper, we describe a fluvial marginal process associated with the formation of iceberg jams in Icelandic proglacial lakes. The floods triggered by the release of these iceberg jams have implications for the geomorphic evolution of the proglacial fluvial system. The process of iceberg jam floods share some conceptual characteristics with Self-Organized Criticality (SOC) approach of complex systems. Using a simple numerical model and field observations, we test the hypothesis that iceberg jam floods exhibit SOC. Field observations and aerial photo-interpretations in southeastern Iceland demonstrate the occurrence of icebergs jam in ice-contact lakes. The mapping of the south Vatnajökull margins between 2003 and 2012 reveals an increase of the calving potentiality and a rise in the likelihood of iceberg jam flood occurrence. Based on the results of the numerical model and field observations, we suggest that iceberg jam floods should be recognized as a SOC phenomenon. Analysis of the simulated time-series show that the iceberg jam floods become less frequent and more similar in magnitude over time. This global trend is related to the gradual enlargement of the lake outlet channel

Microbial diversity associated with the hydrothermal shrimp Rimicaris exoculata gut and occurrence of a resident microbial community

En libre-accès sur Archimer : http://archimer.ifremer.fr/doc/00000/11142/7919.pdfInternational audienceRimicaris exoculata dominates the megafauna of several Mid-Atlantic Ridge hydrothermal sites. Its gut is full of sulphides and iron-oxide particles and harbours microbial communities. Although a trophic symbiosis has been suggested, their role remains unclear. In vivo starvation experiments in pressurized vessels were performed on shrimps from Rainbow and Trans-Atlantic Geotraverse sites in order to expel the transient gut contents. Microbial communities associated with the gut of starved and reference shrimps were compared using 16S rRNA gene libraries and microscopic observations (light, transmission and scanning electron microscopy and FISH analyses). We show that the gut microbiota of shrimps from both sites included mainly Deferribacteres, Mollicutes, Epsilon- and Gammaproteobacteria. For the first time, we have observed filamentous bacteria, inserted between microvilli of gut epithelial cells. They remained after starvation periods in empty guts, suggesting the occurrence of a resident microbial community. The bacterial community composition was the same regardless of the site, except for Gammaproteobacteria retrieved only in Rainbow specimens. We observed a shift in the composition of the microbiota of long-starved specimens, from the dominance of Deferribacteres to the dominance of Gammaproteobacteria. These results reinforce the hypothesis of a symbiotic relationship between R. exoculata and its gut epibionts

Archaeal Methane Cycling Communities Associated with Gassy Subsurface Sediments of Marennes-Oléron Bay (France)

En libre-accès sur Archimer : http://archimer.ifremer.fr/doc/2009/publication-6165.pdfInternational audienceIn Marennes-Oleacuteron Bay, a macro-tidal bay located on the French Atlantic coast, kilometer-scale acoustic turbidity reveals an accumulation of free gas in the sediment. Large concentrations of organic matter and rapid sedimentation rates provide ideal settings for biogenic methane cycling. We integrate seismic, sedimentologic, biogeochemical and molecular genetic approaches to determine whether microbial methane cycling is involved in this process. Here we show that the acoustic turbidity upper boundary matched with X-ray facies displaying fissures with the highest methane concentrations, demonstrating the existence of methane bubbles in the sediment. 16S rRNA and mcrA gene clone libraries were dominated by sequences affiliated to the three known ANME lineages and to putative methanogens. Sequences related to the marine benthic group B (MBG-B) and miscellaneous crenarchaeotal group (MCG) were also detected. However, the highest methane concentration facies was the only section where active Archaea were detected, using reverse-transcribed rRNA, indicating that these communities were involved either directly or indirectly in the methane cycling process. Moreover, three metabolically active novel uncultivated lineages, related to putative methane cycling Archaea, could be specifically associated to these methane bearing sediments. As methane cycling Archaea are commonly retrieved from deep subseafloor and methane seep sediment, the study of coastal gassy sediments, could therefore help to define the biogeochemical habitats of deep biosphere communities

Extending the sub-sea-floor biosphere

En libre-accès sur Archimer : http://archimer.ifremer.fr/doc/2008/publication-4209.pdfInternational audienceSub-sea-floor sediments may contain two-thirds of Earth's total prokaryotic biomass. However, this has its basis in data extrapolation from ~500-meter to 4-kilometer depths, whereas the deepest documented prokaryotes are from only 842 meters. Here, we provide evidence for low concentrations of living prokaryotic cells in the deepest (1626 meters below the sea floor), oldest (111 million years old), and potentially hottest (~100 degrees C) marine sediments investigated. These Newfoundland margin sediments also have DNA sequences related to thermophilic and/or hyperthermophilic Archaea. These form two unique clusters within Pyrococcus and Thermococcus genera, suggesting unknown, uncultured groups are present in deep, hot, marine sediments (~54 degrees to 100 degrees C). Sequences of anaerobic methane-oxidizing Archaea were also present, suggesting a deep biosphere partly supported by methane. These findings demonstrate that the sub-sea-floor biosphere extends to at least 1600 meters below the sea floor and probably deeper, given an upper temperature limit for prokaryotic life of at least 113 degrees C and increasing thermogenic energy supply with depth

Un Paradis perdu. Restaurer ou les effets de l'intervention humaine sur le cours du temps – à propos de quelques réalités monumentales (France, Cambodge)

Les interventions sur les monuments anciens en milieu rural gagneront à être plus « légères » qu’en milieu urbain, dès lors que la part du temps est prise en considération. En France comme au Cambodge, toute restauration complète appelle d’autres travaux à un rythme plus rapproché, plus difficiles d’exécution, plus importants en moyens et plus impactants sur les architectures et sculptures. Entre le XIIe et le XIXe siècle, les maçonneries des églises romanes d’Auvergne ont été peu entretenues, mais l’épiderme des matériaux de construction a globalement bien tenu. Un siècle après les restaurations du XIXe siècle, du fait de l’utilisation des ciments durs et de matériaux exogènes, la dégradation des parties restaurées et non restaurées impose de nouvelles interventions rapprochées dans le temps, alors que la première période n’avait connu aucune grande phase de restauration, l’entretien seul ayant suffi.La déforestation est quant à elle la principale cause de la dégradation accélérée des maçonneries assemblées à sec des temples khmers, mais on tirera une leçon utile de l’exemple du temple de Ta Prohm à Angkor qui conserve son couvert forestier protecteur. Dès lors que l’exubérance de la végétation est contrôlée, au Cambodge comme en France, un édifice ancien supporte aisément la présence de mousses, lichens et cyanobactéries, voire de graminées, qui enveloppent le site dans un biofilm protecteur. À l’inverse, chaque restauration ou nettoyage déclenche l’installation rapide d’organismes jeunes plus agressifs que les organismes âgés vivant au ralenti et finissant par former une « biopatine » protectrice. Face aux bouleversements industriels, le XIXe siècle a restauré ses édifices emblématiques. Notre siècle doit probablement conserver de manière raisonnée des références monumentales synonymes d’une lente disparition. Pourtant un édifice ayant subi de multiples interventions se corrigeant les unes les autres, produit d’une économie de loisirs, ne produira jamais l’émotion d’une ruine discrètement entretenue dans laquelle il est possible de se perdre, en dehors de tout itinéraire balisé et de toute proposition pédagogique. Le « réensauvagement » de sites ruraux est donc un enjeu qui dépasse « l’absolue nécessité », en réalité toute relative des restaurations.Interventions on ancient monuments in rural areas will benefit from being "lighter" than in urban areas, as long as the time issue is taken into account. In France as in Cambodia, any complete restoration calls for other works at a closer pace, more difficult to carry out, more important in terms of resources and more impacting on architectures and sculptures. Between the 12th and the 19th century, the masonry of Romanesque churches in Auvergne was poorly maintained, but the building materials epidermis has generally held up quite well. A century and a half after the 19th century restorations, due to using hard cements and exogenous materials, degradation of the restored and unrestored parts requires new close in time interventions, whereas the first period had not seen any major restoration phase, maintenance alone did suffice.Deforestation is the main cause of the accelerated degradation of dry-assembled masonry of Khmer temples, but a useful lesson will be learned from the example of Ta Prohm temple in Angkor which retains its protective forest cover. As soon as the vegetation exuberance is controlled, in Cambodia as in France, an old building easily supports the presence of mosses, lichens and cyanobacteria, even grasses, which envelop the site in a protective biofilm. Conversely, each restoration or cleaning triggers the rapid installation of young organisms that are more aggressive than older organisms living in slow motion and eventually forming a protective “biopatin”. Faced with industrial upheavals, the 19th century restored its iconic monuments. Our century must probably preserve in a reasoned way monumental references reflecting a slow disappearance. However, a building that has undergone multiple interventions correcting each other, in a leisure economy, will never produce the emotion of a discreetly maintained ruin in which on can get lost, apart from any marked route and any educational proposal. Rural sites “going back to the wild” is therefore an issue that goes beyond the restorations “absolute – actually all relative – necessity”

Mutations des paysages polaires : l’impact morphologique du recul glaciaire sur les cours d’eau marginaux

International audienc

Modification du style fluvial proglaciaire en contexte paraglaciaire : l’exemple du Morsárjökull

National audienc

Post Little Ice Age changes in the proglacial fluvial pattern of the Morsarjökull (South of Iceland, Vatnajökull)

International audienc