GEOSCAN, résultats de la recherche


TitreIce-rich permafrost observed in Holocene slope deposits from shallow geophysics and a coring program in Pangnirtung, Nunavut, Canada
AuteurCarbonneau, A -S; Allard, M; LeBlanc, A -M
SourceEUCOP4, 4th European Conference on Permafrost, abstracts; 2014 p. 94
Séries alt.Secteur des sciences de la Terre, Contribution externe 20140133
RéunionEUCOP4, 4th European Conference on Permafrost; Évora; PT; juin 18-21, 2014
Lat/Long OENS -66.0000 -65.5000 66.2500 66.0000
Sujetspergélisol; congélation du sol; glace fossile; températures au sol; Holocène; géologie des dépôts meubles/géomorphologie; Cénozoïque; Quaternaire
ProgrammeInfrastructures terrestres, Géosciences de changements climatiques
Résumé(disponible en anglais seulement)
A study of permafrost conditions was undertaken in the Hamlet of Pangnirtung, Nunavut, by the Geological Survey of Canada (GSC) and Université Laval's Centre d¿études nordiques (CEN) to support decision makers in their community planning work. Due to demographic growth, the community needs to expand despite a limited territory affected by thaw unstable ice-rich permafrost. A combination of geophysical and geomorphological survey methods were used, including drilling of permafrost cores in coarse frozen surficial deposits and ground penetrating radar surveys. Laboratory analysis allowed a detailed characterization of permafrost in terms of ice contents and grain size. Cryostratigraphic analysis was done via CT-Scan imagery of frozen cores using medical imaging softwares such as ImageJ. This non-destructive method allows a 3D imaging of the entire core in order to locate the amount of excess ice, determine the volumetric ice content an, also, to interpret the ice-formation processes that took place during freezing of the permafrost. Our map of the permafrost conditions in Pangnirtung illustrates that the dominant mapping unit consists of a surface layer between 1 and 4 meters thick of ice-rich colluvial deposits into which aggradational ice formed syngenetically with accretion of colluviums on slopes. The measured volumetric ice contents as high as 83 % are uncommon for permafrost in coarse soils. Also, buried organic layers and paleosols were found imbedded in this colluvium cover. Radiocarbon dating on 18 layers reveals that colluviation associated with overland-flow during snowmelt took place almost without interruption since the early land emergence about 7080 year BP ago. The majority of the radiocarbon dates correspond to the establishment of organic soil during warm and wet periods, mostly since 2000 BP. In the eastern sector of town, colluviums cover till and a network of ice wedges that were revealed by regularly spaced hyperbolic reflectors on GPR profiles. In the western sector of town the colluviums cover ice-rich marine silt and bedrock. The volumetric epigenetic ice content of the underlying marine sediments can be as high as 99 %. The permafrost in Pangnirtung can be considered as polygenetic as permafrost is epigenetic in the till and marine sediments and syngenetic in the surface colluviums. The community is located on a terrace with a gentle slope towards the fjord. The significant water supply from snow-melt runoff from the mountain side contributes to the development of ice-rich syngenetic permafrost in colluviums. This still active process makes difficult the planning of new infrastructures and housing projects. Nonetheless, consideration should be given to the surface drainage to avoid or minimize as much as possible drainage concentration that could create thermal erosion of the ice-rich permafrost or even worse, erosion and tunneling along ice wedges.
Résumé(Résumé en langage clair et simple, non publié)
L'étude des conditions du pergélisol dans la communauté de Pangnirtung, Nunavut, a révélé qu'une grande étendue du territoire est occupée par un sol riche en glace près de la surface. À certains endroits, le pergélisol riche en glace a été formé et enfoui par le ruissellement soutenu, depuis l'émersion des terres, de l'eau de fonte de la neige. Ce processus qui mène à la formation d'un pergélisol riche en glace est encore aujourd'hui très actif et rend difficile la planification de nouvelles infrastructures. La concentration du ruissellement à la surface d'un sol riche en glace risque de conduire à sa dégradation.