Arctic and Alpine Permafrost

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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.

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• 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.

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• 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 lt 0oC) ground persists
  throughout the year and is commonly referred to
  as permafrost, or perennially cryotic ground.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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

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• 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.

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• 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.

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• 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.

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• 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.

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Photo courtesy of Menalie Grubb
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Photo courtesy of Menalie Grubb
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Photo courtesy of Menalie Grubb
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Pingo
Source Wikipedia
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Palsa
http//en.wikipedia.org/wiki/FilePalsaaerialview.
jpg
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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.

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• 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.

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Mean Annual Cycle of the Components of the
Surface Water Budget, Kuparuk River Basin
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Freshet
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Source Déry et al. (2005), JHM.
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The interplay between snow and permafrost
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• 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

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• 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.

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• 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).

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• 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.

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• 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.

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• 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.

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• 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.

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Source Mann and Schmidt (2003)
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Source Mann and Schmidt (2003)
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Source Mann and Schmidt (2003)
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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.

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• 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.

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• 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.

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Source Anisimov (2006)
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Drunken forest
http//upload.wikimedia.org/wikipedia/commons/7/7f
/20070801_forest.jpg