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.
1
• 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 coldclimate processes are freezing and
thawing of the ground surface.
2
• 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.
3
• 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.
4
• 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.
5
• 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.
6
• 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.
7
8
9
• 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.
10
• Soil and rock do not automatically freeze
at 0oC, especially if percolating groundwater 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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15
• 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.
16
• 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.
17
• 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:
18
• 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 icerich zone to form in the upper few metres
of permafrost.
19
• 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.
20
• 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.
21
• 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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23
Photo courtesy of Menalie Grubb
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Photo courtesy of Menalie Grubb
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Photo courtesy of Menalie Grubb
Pingo
Source: Wikipedia
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Palsa
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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.
28
• 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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30
Mean Annual Cycle of the Components of the
Surface Water Budget, Kuparuk River Basin
292
Freshet
169
127
1
31
Source: Déry et al. (2005), JHM.
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
33
• 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.
34
• 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).
35
• 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.
36
• 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.
37
• 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.
38
• 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)
42
Source: Mann and Schmidt (2003)
43
Source: Mann and Schmidt (2003)
44
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.
45
• 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.
46
• 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
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