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TitlePermafrost-ecological relations within the northern Great Slave region, Northwest Territories, Canada
AuthorMorse, P DORCID logo; Wolfe, S AORCID logo; Kokelj, S V; Gaanderse, A J
SourceCANQUA-CGRG Biannual Meeting, abstracts; 2013 p. 1
Alt SeriesEarth Sciences Sector, Contribution Series 20130143
PublisherUnivesity of Alberta
MeetingCANQUA-CGRG Biannual Meeting; Edmonton; CA; August 18-22, 2013
Mediapaper; digital
File formatpdf (Adobe® Reader®)
ProvinceNorthwest Territories
NTS85J; 85K
AreaBehchoko; Tibbitt Lake; Great Slave Lake
Lat/Long WENS-118.0000 -114.0000 63.0000 62.0000
Subjectssurficial geology/geomorphology; environmental geology; Nature and Environment; permafrost; ground ice; periglacial features; ecology; thermal analyses; thermal regimes; vegetation; climate; organic deposits; peatlands; soils; sediments; temperature; ground temperatures; boreholes; soil moisture; snow; statistical analyses; thermal conductivity; Great Slave Lowland Ecoregion; Great Slave Upland Ecoregion; Climate change; Forests
ProgramClimate Change Geoscience
Released2013 08 01
AbstractThis study reports on the ground thermal regime of permafrost occurring within distinct ecological settings of the Great Slave Lowland and Upland High Boreal Ecoregions, which lie within the extensive discontinuous permafrost zone. Permafrost distribution has commonly been associated with peatlands, but is also associated with coniferous (black-spruce dominant) and deciduous (white-birch dominant) forests, although is generally absent beneath fens, sandy jack-pine forest, and bedrock outcrops. This terrain mosaic leads to abrupt transitions in the occurrence of permafrost that are associated with changes in ground ice content. Disturbance or warming climate can thaw permafrost, leading to terrain instability if there are high ground-ice contents; however the potential response of permafrost is not well understood as there are few data on variation of the ground thermal regime in this region.
A 2010-13 field program was conducted to examine permafrost conditions at undisturbed peatland and spruce and birch forest sites on a 170 km transect of the Great Slave Lowland and Upland, along the north shore of Great Slave Lake, between Behchoko and Tibbitt Lake. Field data included eight air temperature sites, 19 near-surface ground temperature sites, nine deep ground temperature sites, 14 boreholes, ecological descriptions at 48 sites (vegetation cover, active-layer thickness, soil moisture, and organic-layer thickness), and 50 snow depth sites. The 1981-2010 mean annual air temperature at Yellowknife Airport was -4.1°C. At the study sites, annual mean air temperature was -4.3°C in 2010-11, but was 2°C higher in 2011-12 mainly due to higher winter air temperatures. Regression analysis of air temperature data indicated regionally consistent air temperatures, and a +0.5°C bias at Yellowknife Airport. Annual mean surface temperatures (2010-12) ranged from -0.5 to 3.2°C, and were about 1°C higher in peatlands than at forested sites. Conversely, annual mean temperatures at 100-cm depth were at least 1°C lower in peatlands than at forested sites, and ranged overall from -3.7 to -0.1°C. Permafrost occurred at sites with thermal offsets ranging from -6.1 to 0.1°C. Annual mean ground temperatures (5 to 10 m depths) at peatlands and forested sites ranged from -0.98 to -0.42°C and each site was approximately isothermal. Compared with peatland sites, deciduous forest sites had deeper, drier active layers with thin surface organic layers. Data for coniferous forest sites was intermediate between those of peatlands and deciduous forest. Median late-March (2013) snow depths ranged from 39 cm in deciduous forest to 47 cm at peatlands, close to the Yellowknife median annual late-winter maximum snow depth of 45 cm.
On an annual basis, ground-surface temperature variation was influenced mainly by differences in substrate materials, particularly the moisture content and thermal conductivity of the freezing/thawing layer. Higher surface temperatures at peatlands were due mainly to freezing season conditions of the active layer, with greater than 1.5 times the amount of latent heat released from freezing the soil compared with forested sites, but once frozen, icy peat has a high thermal conductivity that facilitates ground cooling. Conversely, the average winter surface temperatures at forested sites were lower, likely due to drier soil and less latent heat released from active-layer freezing. At forested sites, temperatures at 100-cm depth remained close to 0°C, likely due to the relatively low thermal conductivity of the active layer and high thermal inertia associated with phase change of unfrozen water content at the top of permafrost. As the permafrost beneath forested sites was isothermal, increased surface temperature would direct energy toward thawing, rather than warming. Preliminary data suggest thermal conditions differ amongst the various soil and vegetation types in the region, and indicate the possibility of combining field data and remote sensing to create predictive maps of permafrost conditions.
Summary(Plain Language Summary, not published)
We report on ground temperatures in permafrost terrain (ground that remains frozen year-round) within different ecological settings in the Great Slave region, Northwest Territories. Whereas near-surface ground temperatures in peatlands are warmer than in forests, deeper (e.g. 100-cm depth) ground temperatures are colder. We associate the differences in ground temperatures to the moisture content and thermal conductivity (ability of the material to transmit heat) of the soils. These data imply that, with warming, forested sites may be more susceptible to thawing than peatlands. Our results may further be used to develop predictive maps of permafrost conditions in this region.

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