|Résumé||(Sommaire disponible en anglais seulement)|
Climate change can have an impact on urban development and infrastructure in the Arctic as permafrost temperature and active layer thickness increase
and, also, as more frequent extreme climatic events trigger landscape hazards. The Canada-Nunavut Geoscience Office (CNGO) launched the "Nunavut Landscape Hazard Mapping Initiative" in 2009 to provide geoscience information and expertise to Nunavut
communities in order to support them in developing adaptation and management strategies that incorporate anticipated climate changes. Under this initiative, a study of permafrost conditions was undertaken in the Hamlet of Pangnirtung by
Geological Survey of Canada (GSC) and Université Laval's Centre d'études nordiques (CEN).
Pangnirtung was seriously affected by an extreme rain event in June 2008, which has led to severe permafrost degradation along the Duval River banks
due to thermal erosion. On a community scale, ground investigations remain the most effective way of characterizing permafrost conditions. A combination of mapping, shallow and deep drilling, sampling, shallow geophysical surveys, ground thermal
instrumentation, and snow surveys was used during the summer 2009 and winter 2010 field seasons. This Open File outlines the methodology used in the field and presents current permafrost conditions based on the results of the 2009/10 investigation,
including the surficial geology, ground ice content and ground thermal regime of Pangnirtung. These results form the baseline information needed to assess the landscape hazards and permafrost sensitivity to climate warming or any human disturbances
and, therefore, support community planning. Results are not only targeted to the community of Pangnirtung, but also to engineers and land use planners working in the North, since the methodology and results can be transferable to other northern
communities and Arctic environments.
Prior to the field work, airphoto interpretation was conducted to identify the surficial geology and delineate landscape features associated with permafrost. The airphoto interpretation was reassessed in the
field. During summer 2009, pits were dug to the thaw front and the underlying permafrost was cored with a portable earth drill. The samples were kept frozen for laboratory analysis (ice and water contents, salinity, grain size, Atterberg limits, and
radiocarbon dating). In addition, two deep (~15 m) holes were drilled with an airtrack drill. Thermistor cables were installed in the deep boreholes and in two additional shallow (~3 m) boreholes to monitor the permafrost temperatures in different
terrain units. These cables supplement a cable that was installed by the GSC the previous year. Miniature loggers equipped with a single temperature sensor were buried in the near surface (~5 cm) at various sites across the village to monitor the
soil surface temperatures and, therefore, provide information on the surface thawing and freezing indices. Two types of shallow geophysical surveys were conducted in the study area: ground penetrating radar and electrical resistivity using both a
capacitively-coupled resistivity meter and a multi-electrode galvanic resistivity meter. These data are more extensive, spatially continuous, and provide deeper subsurface information than the mapping and drilling data. Finally, in March 2010, the
snow cover was characterized in order to correlate snow properties and soil surface temperatures.
Four main areas of Pangnirtung, distinguished by their unique terrain features, were first identified by airphoto interpretation and thereafter
investigated in the field (see Figure 12): 1) the alluvial fan through which the Duval River flows, 2) the alluvial terrace with boulders and eroded channels along the banks of the Duval River, 3) the till terrace covered by colluvium to the east of
the alluvial fan, also known to be the next development area of the community, and 4) the marine sediments covered by colluvium to the west of the alluvial fan. Results are shown on: 1) the distribution of ice-rich permafrost and its relationship to
the surficial geology, 2) the thickness of the surficial and underlying layers and, 3) the different ground thermal regime in each distinctive terrain type. An icerich colluvium blanket was found throughout the studied areas, but mostly in the
sectors to the west and east of the town. On the east side of the Duval River, in addition to the ice-rich colluvium, a network of ice wedges may exist in the underlying till. The thickness of the colluvium blanket is estimated to be between 1 and 3
m and is probably discontinuous over the area. The thickness of the marine sediments appears quite variable, but generally appears to increase from west to east, especially on the south side of the runway. The thickness of the alluvial fan varies
from about 2 to 10 m with the greater thickness appearing to be on the south-west portion of the alluvial fan. The alluvial fan overlies a till deposit as seen in the eroded bluff face, except close to the coastline near the mouth of the Duval River
where sandy, silty and shell-rich marine sediments were found underlying the alluvial fan deposit. The thickness of the till to the east of the alluvial fan is at least 15 m, since bedrock was not encountered during drilling.
warmer in the vicinity of the alluvial fan and colder away from it to the east and west. At 12 m depth, the ground temperatures are -2.8, -5.2, and -7.1 ºC in the alluvial fan, in the marine sediments at the airport, and in the till terrace to the
east, respectively. In the marine sediments, the maximum thaw depth in 2009 varied from 0.75 m to 1.8 m (measured at three locations), while in the till terrace and the alluvial fan it was about 1 and 2.5 m, respectively. Surface thawing indices
varied from 725 (natural conditions) to 1263 (gravel road) degree days, while freezing indices ranged from 756 (thick snow cover) to 3029 (bare road) degree days. Such difference in sediment types, ice content and permafrost temperatures will
eventually lead to a spatially variable thermal and physical response of the permafrost to climate warming. The area along the river currently represents the most vulnerable geological sector in the community. The permafrost is warmer there than in
any other area in the community, especially where snow drifts accumulate along the river bank. Coarse grained material, typical of the alluvial fan, exhibits a greater thermal response to changes in air temperature than fine-grained material, and
therefore, the ground thermal regime of this area is expected to respond more rapidly to climate warming. The physical response (i.e. ground subsidence) has also proved to be high close to the river due mainly to the underlying till, which consists
of a fine-grained matrix easily eroded during high discharge of the Duval River. Although the other areas studied have colder permafrost, the presence of ice-rich permafrost within the colluvium cover could eventually lead to thaw settlement if the
active layer thickness increases. Possible ice wedges in the till terrace add to the sensitivity of this area to climate warming or human disturbances. Some care must be taken not to channel surface water flow into frost cracks in order to prevent
thermal erosion and gullying. A complete assessment of the landscape hazards related to permafrost degradation and a more detailed study of the thermo-erosion processes along the Duval River are underway. These will provide further tools in support
of the economic development through infrastructure maintenance and community planning in Pangnirtung.