Page 587 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

soils exceed any increases in carbon uptake that the warmer conditions and higher carbon dioxide levels create, making permafrost thaw an important source of greenhouse gases. (Please review the Chapter 11 Geo- systems Now.)
Permafrost Behaviour Figure 17.20 shows a cross section of a periglacial region in northern Canada, extending from approximately 75° N to 55° N through the three sites located on the map in Figure 17.19. The zone of seasonally frozen ground that exists between the subsurface permafrost layer and the ground sur- face is called the active layer and is subjected to con- sistent daily and seasonal freeze–thaw cycles. This cyclic thawing of the active layer affects as little as 10 cm of depth in the north of the periglacial region (Ellesmere Island, 78° N), up to 2 m in the southern margins (55° N) of the periglacial region, and 15 m in the alpine permafrost of the Colorado Rockies (40° N).
Resolute Kugluktuk
Hotchkiss
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Permafrost actively adjusts to changing climatic conditions: Higher temperatures reduce per- mafrost thickness and increase the thickness of the active layer; lower temperatures gradually increase permafrost thickness and reduce active-layer thickness. Although somewhat sluggish in response, the active layer is a dynamic, open system driven by energy gains and losses in the subsurface environment.
Subsea permafrost
Continuous permafrost
Discontinuous permafrost
Alpine permafrost
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With the warming tempera-
tures recorded in the Canadian
and Siberian Arctic since 1990,
more disruption of permafrost surfaces is occurring— leading to highway, railway, and building damage. In Siberia, many lakes have disappeared in the discon- tinuous permafrost region as thawing of permafrost opens the way for subsurface drainage; yet in the continuous region, new lakes have formed as a result of thawed soils becoming waterlogged. In Canada, hundreds of lakes have disappeared from excessive evaporation into the warming air. These trends are measurable from satellite imagery.
Periglacial Processes
In regions of permafrost, frozen subsurface water is ground ice. The amount of ground ice present varies with moisture content, ranging from only a small per- centage in drier regions to almost 100% in regions with saturated soils. The presence of frozen water in the soil initiates geomorphic processes associated with frost action.
Frost Action Processes The 9% expansion of water as it freezes produces strong mechanical forces that fracture rock and disrupt soil at or below the surface. If sufficient water freezes, the saturated soil and rocks are subjected to frost heaving (vertical movement) and frost thrusting (horizontal movement). Boulders and rock slabs may be thrust to the surface. Soil horizons (layers) may be disrupted by frost action and appear to be stirred or churned. Frost action also can pro- duce contractions in soil and rock, opening up cracks in which ice wedges can form.
A talik is an area of unfrozen ground that may occur above, below, or within a body of discontinu- ous permafrost or beneath a water body in regions of continuous permafrost. Taliks occur beneath deep lakes and may extend to bedrock and noncryotic soil beneath large, deep lakes (see Figure 17.20). Taliks in areas of discontinuous permafrost form connections between the active layer and groundwater, whereas in continuous permafrost, groundwater is essentially cut off from surface water. In this way, permafrost disrupts aquifers and taliks, leading to water-supply problems.
Chapter 17 glacial and Periglacial Landscapes 551
▲Figure 17.19 Permafrost distribution. Distribution of permafrost in the northern Hemi- sphere. alpine permafrost is noted except for small occurrences in Hawai’i, Mexico, europe, and Japan. Subsea permafrost occurs in the ground beneath the arctic Ocean along the margins of the continents, as shown. note the towns of resolute and Kugluktuk in nunavut and Hotchkiss in alberta. a cross section of the permafrost beneath these towns is shown in Figure 17.20. [adapted from T. L. Péwé, “alpine permafrost in the contiguous United States: a review,” Arctic and Alpine Research 15, no. 2 (May 1983): 146. © University of Colorado. Used by permission.]