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Concept

Chaîne de montagnes

Résumé

vignette|Vue satellitaire de la chaîne des Alpes. vignette|Vue satellitaire de la partie sud de la cordillère des Andes. vignette|Vue de la chaîne de l'Himalaya. vignette|Vue aérienne d'une ligne de crête dans la cordillère des Andes. droite|vignette|Les dorsales océaniques sont représentées par une ligne rouge. Une chaîne de montagnes est un relief d'altitude rassemblant plusieurs montagnes (sommets, pics, monts et autres aiguilles) et présentant une longueur beaucoup plus grande que sa largeur. Une chaîne particulièrement longue et étroite est qualifiée de cordillère. L'origine géologique d'une chaîne de montagnes s'explique par la tectonique des plaques. Elle est la conséquence soit de la collision de deux plaques continentales, soit de la subduction d’une plaque océanique sous une plaque continentale ou sous une autre plaque océanique. Les chaînes de montagnes sont des éléments géomorphologiques de premier plan. Elles influent sur le climat, sur la répartition des populations ani

Source officielle

https://fr.wikipedia.org/wiki/Cha%C3%AEne_de_montagnes

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Chargement

Theory predicts that galaxies are not randomly distributed in the Universe. They form a complex network of filamentary structures, the Cosmic Web, divided in clusters, groups, filaments and walls. This picture has been confirmed by the large galaxy redshift surveys. Galaxy properties like morphology and star formation rates are strongly influenced by their environment. The most radical transformations are observed in galaxy clusters, located at the intersection of filaments. Nevertheless, the location where the environment starts to play a role and the main physical phenomenon responsible for these changes are still unknown. The aim of this thesis is to determine where and how galaxies are affected by the environment, at local and global scales. We gathered high quality photometry in the 5 optical bands

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with MegaCam on the Canada-France-Hawaii Telescope for 3 medium mass galaxy clusters at intermediate redshift, with a spatial extent on the sky plane reaching 8 to 13 times the cluster virial radii. These clusters were selected to represent as much as possible progenitors of clusters in the local Universe. The core of these clusters have rich ancillary data from the ESO Distant Cluster Survey. The redshift and mass ranges of these clusters, combined to the wide field of the observations make this study unique. We computed photometric redshifts to map the galaxy distribution around the clusters. We were able to identify overdense regions forming groups and filaments, unveiling for the first time the large scale structures in such details for this type of clusters. We studied the fraction of galaxies in the red sequence in different environments, namely in the field, filaments and groups. We find that significant quenching processes are already at work in filaments, even at large cluster-centric distances. We also show that the local neighborhood of galaxies has a stronger impact on galaxy colors than the global environment. An optical spectroscopic follow-up with the VIsible MultiObject Spectrograph on the Very Large Telescope confirms our photometric results. In particular, the fraction of galaxies with detected \otwo emission line is smaller with increasing density. We also find that the star formation rate, determined from [OII], is in average weaker in groups and clusters than in lower density environments. This thesis opens new perspectives to study the way galaxies populate the Cosmic Web. Combined to weak-lensing analysis and cosmological simulations, it will allow a deeper understanding of the structure formation and their baryonic content. It is a first step before extending the results to much larger samples, with the help of the upcoming all-sky cosmological surveys such as Euclid or the Square Kilometer Array.

EPFL2015

Chargement

From the clouds to the ground - Effects of flow-precipitation interactions on snow distribution in complex alpine terrain

Franziska Gerber

Knowledge about the spatial distribution of seasonal snow is essential e.g. to efficiently manage fresh water resources or for hydro-power companies. The large-scale gradient of snow accumulation over mountain ranges is mainly determined by lifting condensation and downstream drying. On the slope-scale, snow redistribution by wind and avalanches is the main source of variability. On a mountain-ridge to mountain-valley scale, small-scale orographic precipitation enhancement and preferential deposition interact and lead to asymmetric snow distribution across mountain ridges. However, their relative importance is barely known and the characteristics of preferential deposition are still under debate. Yet, especially in a changing climate, which may go along with modified dominant wind directions, it is important to understand precipitation processes shaping the snow cover. Therefore, we investigate terrain-flow-precipitation interactions and their effect on mountain-ridge to mountain-valley scale snow precipitation and deposition in complex alpine terrain. To this end, the Weather Research and Forecasting (WRF) model is set up to downscale Consortium for Small-Scale Modeling (COSMO) analysis to a horizontal resolution of 50 m using a nesting approach. At 450 m resolution these simulations fairly represent large-scale precipitation variability with respect to high-resolution operational weather radar precipitation estimates, capturing the effect of large-scale orographic enhancement. Although, the model misses substantial small-scale precipitation variability even at a 50 m resolution, we demonstrate that the lee-side flow field and mountain-ridge scale precipitation processes start to be represented at this resolution. Thus, a model resolution of at least 50 m is required to represent mountain-ridge to mountain-valley scale precipitation patterns, which is far higher than model resolutions conventionally used to simulate snow water resources in alpine regions. Small-scale orographic precipitation enhancement and mean advection are estimated to increase lee-side precipitation by up to 20%, while a conservative estimate of (near-surface) preferential deposition reveals lee-side snow deposition enhancement on the order of 10%. However, both processes strongly depend on atmospheric conditions such as atmospheric humidity or the strength of mean advection. The peculiarity of the lee-side flow field is of particular importance for the spatial distribution of snow accumulation, especially with regards to preferential deposition. This is further demonstrated by a very persistent eddy-like structure on the leeward side of the Sattelhorn ridge in the Dischma valley (Davos, CH), as reported based on Doppler wind lidar measurements and with corresponding flow field simulations at a resolution of 25 m by the Advanced Regional Prediction System (ARPS). Corresponding snow accumulation, assessed by the means of terrestrial laser scanning, confirms that snow distribution in very steep terrain is strongly influenced by post-depositional snow redistribution. Nevertheless, we can report a certain agreement of simulated pre-depositional precipitation patterns across mountain ridges with photogrammetrically determined snow distribution. Overall, we demonstrate the necessity and value of high-resolution snow precipitation measurements and simulations, and we contribute to the understanding of the small-scale variability of snow distribution in alpine terrain.

EPFL2018

Chargement

Benthic Biofilms in Glacier-Fed Streams from Scandinavia to the Himalayas Host Distinct Bacterial Communities Compared with the Streamwater

Tom Ian Battin, Massimo Bourquin, Jade Brandani, Susheel Bhanu Busi, Vincent Henri De Staercke, Leïla Ezzat, Stylianos Fodelianakis, Tyler Joe Kohler, Grégoire Marie Octave Edouard Michoud, Emmy Marie Oppliger, Hannes Markus Peter, Paraskevi Pramateftaki, Martina Andrea Schön, Michail Styllas, Virginia Tadei, Matteo Tolosano

Microbial life in glacier-fed streams (GFSs) is dominated by benthic biofilms which fulfill critical ecosystem processes. However, it remains unclear how the bacterial communities of these biofilms assemble in stream ecosystems characterized by rapid turnover of benthic habitats and high suspended sediment loads. Using16S rRNA gene amplicon sequence data collected from 54 GFSs across the Himalayas, European Alps, and Scandinavian Mountains, we found that benthic biofilms harbor bacterial communities that are distinct from the bacterial assemblages suspended in the streamwater. Our data showed a decrease in species richness in the benthic biofilms compared to the bacterial cells putatively free-living in the water. The benthic biofilms also differed from the suspended water fractions in terms of community composition. Differential abundance analyses highlighted bacterial families that were specific to the benthic biofilms and the suspended assemblages. Notably, source-sink models suggested that the benthic biofilm communities are not simply a subset of the suspended assemblages. Rather, we found evidence that deterministic processes (e.g., species sorting) shape the benthic biofilm communities. This is unexpected given the high vertical mixing of water and contained bacterial cells in GFSs and further highlights the benthic biofilm mode of life as one that is determined through niche-related processes. Our findings therefore reveal a "native" benthic biofilm community in an ecosystem that is currently threatened by climate-induced glacier shrinkage. IMPORTANCE Benthic biofilms represent the dominant form of life in glacier-fed streams. However, it remains unclear how bacterial communities within these biofilms assemble. Our findings from glacier-fed streams from three major mountain ranges across the Himalayas, the European Alps and the Scandinavian Mountains reveal a bacterial community associated with benthic biofilms that is distinct from the assemblage in the overlying streamwater. Our analyses suggest that selection is the underlying process to this differentiation. This is unexpected given that bacterial cells that are freely living or attached to the abundant sediment particles suspended in the water continuously mix with the benthic biofilms. The latter colonize loose sediments that are subject to high turnover owing to the forces of the water flow. Our research unravels the existence of a microbiome specific to benthic biofilms in glacier-fed streams, now under major threats due to global warming. Benthic biofilms represent the dominant form of life in glacier-fed streams. However, it remains unclear how bacterial communities within these biofilms assemble.

AMER SOC MICROBIOLOGY2022

Chargement

From the clouds to the ground - Effects of flow-precipitation interactions on snow distribution in complex alpine terrain

Franziska Gerber

EPFL2018

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EPFL2015

Benthic Biofilms in Glacier-Fed Streams from Scandinavia to the Himalayas Host Distinct Bacterial Communities Compared with the Streamwater

AMER SOC MICROBIOLOGY2022