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Arctic and Alpine Permafrost. Definition: Permafrost is a layer of permanently frozen ground, that is, a layer in which the temperature has been continuously below 0 o C for at least two years.
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Arctic and Alpine Permafrost • Definition: Permafrost is a layer of permanently frozen ground, that is, a layer in which the temperature has been continuously below 0oC for at least two years. • This means that moisture in the form of either water or ice may or may not be present. Permafrost may therefore be unfrozen, partially frozen, or frozen depending on the state of the ice/water content.
Seasonally frozen ground, or active layer, is usually a layer above the permafrost that freezes in winter and thaws in summer where depth of thawing from the surface is usually less than a metre or so in thickness. • Central to the operation of most cold-climate processes are freezing and thawing of the ground surface.
These may occur either diurnally, as in many temperate and subtropical regions, or seasonally, as in much of northern Canada. • The depth of frost penetration depends mainly on the intensity of the cold, its duration, thermal and physical properties of the soil and rock, and overlying vegetation. • Where the depth of seasonal frost exceeds that of thaw during the summer following, a zone of frozen (i.e. temperature < 0oC) ground persists throughout the year and is commonly referred to as permafrost, or perennially cryotic ground.
All three conditions - diurnal frost, seasonal frost, and permafrost - influence the nature and extent of cold-climate processes. • The seasonal (i.e. annual) rhythm of ground freezing and thawing dominates much of northern Canada where long, cold winters are typical. • Usually, spring thaw occurs quickly and over three-quarters of the soil thaws during the first four to five weeks in which air temperatures are above 0oC. Ground thermal regimes are closely related to snow thickness and density.
Autumn freeze-back is equally complex - in regions underlain by continuous permafrost, freezing is two-sided, occurring both downward from the surface and upward from the perennially frozen ground beneath, and the freezing period is much longer and may persist for 6 to 8 weeks. • During most of this period the soil remains in a near-isothermal conditions as a result of the release of latent heat on freezing that delays the drop in temperature.
Permafrost is found in the Arctic and subarctic, in high mountain ranges, and in ice-free regions of Antarctica. • There is broad zonation of permafrost conditions in Canada according to climate. • Zones of either continuous or discontinuous permafrost are recognized, in addition to alpine permafrost or subsea permafrost.
In total, approximately 50% of Canada's land surface is underlain by permafrost of some sort. • The southern limit of the zone of continuous permafrost correlates well with the approximate position of the -6 to -8oC mean annual air temperature isotherm, and this relates to the -5oC isotherm of mean annual ground temperatures. • The discontinuous zone is further subdivided into areas of widespread permafrost and scattered permafrost; at its extreme southern fringes, permafrost exists as isolated “islands” beneath peat and other organic sediments.
In certain areas of the western Canadian Arctic underlain by unconsolidated sediments, ground ice may comprise at least 50% by volume of the upper 1-5 m of permafrost. • Although many types of ground ice can be recognized, pore ice, segregated ice, and wedge ice are the most significant in terms of volume and widespread occurrence. • There is a tendency to regard a frozen soil as one in which the water has been replaced by ice; in fact, at most temperatures of interest, frozen soils contain ice and water.
Soil and rock do not automatically freeze at 0oC, especially if percolating ground-water is highly mineralized or under pressure. • As a result, significant quantities of unfrozen porewater may continue to exist at temperatures below zero. • The more fine-grained a soil is, the greater is the amount of water remaining at a given temperature.
As the water content is reduced by progressive formation of ice, the remaining water is under an increasing suction that develops by freezing. • Intimately associated with ground freezing are the phenomena of frost heaving and ice segregation, which take place wherever moisture is present within the soil. • Frost heaving caused by ice segregation occurs throughout much of Canada.
Annual ground displacements of several centimeters are common, with cyclic differential ground pressures of many kilopascals per square centimeter. • Field studies in the Mackenzie Delta region indicate that heave occurs not only during autumn freeze-back, but also during winter when ground temperatures are below 0oC. • Geomorphic evidence of frost heaving include upheaval of bedrock blocks, upfreezing of objects and tilting of stones, and sorting and migration of soil particles.
Engineering hazards caused by these displacements and pressures, together with adverse effects of accumulations of segregated ice in freezing soil, are widespread and costly. • For instance, foundations for roads and pipelines in permafrost regions require large quantities of coarse grained materials to reduce the heaving during winter. • There are 3 major considerations related to the water/ice content of permafrost:
1) The freezing of water in the active layer at the beginning of winter each year results in ice lensing and ice segregation. The amount of heave will vary according to the amount and availability of moisture in the active layer, with poorly drained silty soils showing the maximum heave effects as unfrozen water progressively freezes. This moisture migrates in response to a temperature gradient and causes an ice-rich zone to form in the upper few metres of permafrost.
2) Ground ice is a major component of permafrost, particularly in unconsolidated sediments. If ground ice-rich permafrost thaws, subsidence of the ground results. A range of processes are associated with permafrost degradation are summarized under the term “thermokarst”. • 3) The hydrological and groundwater conditions of permafrost terrain are unique. Subsurface flow is restricted to unfrozen zones called taliks and to the active layer.
These are three groups of features whose formation necessarily involve permafrost and which therefore are diagnostic of permafrost conditions: a) patterned ground, including ice wedge polygons, stone polygons, sorted circles, sorted stripes, and nonsorted circles; b) palsas, and c) pingoes. • Permafrost terrain is generally regarded as highly sensitive to thermal disturbance.
Mapping permafrost is not a straightforward endeavour as remote sensing instruments are capable of sensing freeze-thaw processes only within the uppermost 5 cm of soil depth. • The spatial correlation length of permafrost variability is linked to the surface vegetation and soil type plus the volumetric water content of the soil. • Most of the Canadian north is characterized by permafrost soils at temperatures greater than -2oC with frozen thicknesses less than 75 m.
Pingo Source: Wikipedia
Palsa http://en.wikipedia.org/wiki/File:Palsaaerialview.jpg
Active Layer • Between the upper surface of permafrost and the ground surface lies the active layer, a zone that thaws each summer and refreezes each autumn. • In thermal terms, it is the layer that fluctuates above and below 0oC during the year. Its thickness varies from as little as 15-30 cm in the High Arctic to over 1.5 m in the Canadian subarctic.
Thickness depends on many factors, including ambient air temperatures, angle of slope and orientation, vegetation cover, thickness (depth and density) and duration of snow cover, soil and rock type, and ground moisture conditions.
Mean Annual Cycle of the Components of the Surface Water Budget, Kuparuk River Basin 292 169 Freshet 127 1 Source: Déry et al. (2005), JHM.
Ground temperatures are strongly influenced by conductive heat transfer, although localized circulation of groundwater can occur, particularly in areas of discontinuous permafrost. • Under steady-state conditions, the mean annual ground temperature profile is linear with depth (assuming constant thermal conductivity), and temperature at any depth Tz is given by: • Tz = Ts + Gz
Where Ts is surface temperature and G is geothermal gradient (increase in temperature with depth within the ground). • In reality, heat conduction in the ground is more complex: steady states are rarely achieved, since surface temperature is continually changing, and natural variations in soil conditions leads to differences in thermal properties. In addition, thermal properties of frozen soils vary with temperature.
The thermal regime in the upper layers of the ground is controlled by exchanges of heat and moisture between the atmosphere and Earth's surface. • The processes involved in the energy balance comprise net exchange of radiation (Q*), between surface and atmosphere, transfer of sensible (QH) and latent heat (QE) by the turbulent motion of the air, and conduction of heat into the ground (QG).
Partitioning of the radiative surplus (or deficit) among the heat fluxes is governed by the nature of the surface and the relative abilities of the ground and the atmosphere to transport heat energy. • Each term affects surface temperature, and thus the way in which the energy balance is achieved establishes the surface temperature regime.
Snow profoundly affects the ground thermal regime, since it presents a barrier to heat loss from the ground to the air. • In the Mackenzie delta, where mean daily air temperature is below -20oC for almost six months in winter, the 1-m ground temperature beneath 120 cm of snow did not fall below -0.2oC. • In marginal areas of permafrost distribution, snow cover alone may be the critical local factor determining the presence of permafrost.
In the colder regions of more widespread permafrost, it influences the depth of the active layer. • Also, in regions of heavy snowfall, lake and river ice will not be so thick, so that even bodies with shallow water may not freeze through, as in the Mackenzie delta where snow cover shapes local distribution of permafrost. • A study by Goodrich (1982) shows that doubling of snow cover from 25 to 50 cm increased minimum ground surface temperature by about 7oC and mean annual surface temperature by 3.5oC.
If the 50 cm of snow accumulates within thirty days in autumn, mean temperature would rise above 0oC and permafrost would degrade. • Precipitation increases of as much as 60% in autumn and early winter projected in some climate models would therefore help accelerate permafrost degradation, particularly in marginal areas.
Degradation • Degradation of permafrost often involves melting of ground ice accompanied by local collapse and subsidence of the ground. • These processes are termed thermokarst, a physical (i.e. thermal) process peculiar to permafrost regions.
Since thermokarst merely reflects a disruption in the thermal equilibrium of the permafrost, a range of conditions can initiate it, including changes in regional climate, localized slope instability and erosion, drainage alteration, and either natural (i.e. fire) or human-induced disruptions to surface vegetation cover. • In the boreal forest, fire frequently initiates permafrost degradation and slope failure.
Along the western arctic coastal plain, where alluvial sediments with high ice contents are widespread, thermokarst is believed to be one of the principal processes fashioning the landscape. • Elsewhere, large-scale thermokarst phenomena include ground-ice slumps and thaw lakes.
