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Soil in Cold Climates
Thermal characteristics of cold soil
The most striking thermal characteristics of permafrost-affected
land are the low soil temperatures, a perennially frozen portion of
the subsoil and a pronounced change in temperature with depth.
In many Arctic regions, the temperature of the topsoil can drop to
an astonishing -30°C during the winter while not exceeding 10°C
during the summer months. These harsh thermal conditions reduce
biological activity, rates of chemical processes and weathering.
The annual temperature gradient (unfrozen to perennially frozen)
provides the mechanism for water to move in the soil, for ground
ice to build up and for cryogenic processes to operate. Although
soil temperatures are directly related to air temperature, factors
such as vegetation cover, soil moisture content, texture, thickness
of snow cover and underlying permafrost have a modifying effect.
The figure below shows the annual fluctuation of soil temperatures
measured on Ellesmere Island in the High Arctic of Canada. Topsoil
temperatures reflect fluctuating air temperatures. Changes in the
temperature of the subsoil are less pronounced. During the summer
months, the temperature of topsoil climbs above 0°C causing the
ice to thaw. However, during the same period the subsoil remains
frozen.
During the summer, the temperature of the soil is less than that of
the air due to the cooling effect of the permafrost. However, in
the winter the opposite situation occurs with the soil being slightly
‘warmer’ than the air temperature.
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This profile from a thaw lake basin in northern Alaska shows a soil that is frozen below a depth of 35
cm with unfrozen material (active layer) above. On the surface a 15 cm limnic horizon (associated
with a submerged environment) which overlies a marl (lime-rich mud) layer at 15-22 cm. Below 40
cm, pure ice is visible which does not melt during the summer. The irregular dark patterns below
40 cm are due to cryoturbated organic material mixing with the gray mineral soil material. Well
developed vein ice, seen at 80 cm, gives rise to the horizontal structure in the profile. (CP)
Temperature (°C)
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-1
-20
-30
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March
May
July
Sept
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Air
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Mean monthly air and soil temperatures measured at depths of
20 cm, 50 cm and 100 cm in 2004 at a monitoring station near
Lake Hazen on Ellesmere Island in the High Arctic of Canada
(Lat. 81° 49' 15" N; Long. 71° 33' 17" W). The graph shows the
summer heating of topsoil which leads to the seasonal thawing
and the development of an active layer, which in this site is no
more than 50 cm thick. (CT)
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30 t, min
The weathering of parent material is the main mechanism to make
essential elements available to plants. This availability defines soil
fertility that drives the productivity of ecosystems.
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Because of the low temperatures, chemical and biological weathering
(the destruction of rocks and minerals) in northern latitudes is
generally limited. However, physical weathering (the disintegration
of solid rock into smaller fragments) is often intense.
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t, °C
Freezing soil
When water in the soil begins to freeze, ice crystals form. However,
not all the water immediately turns to ice. The temperature at the
interface between the ice and still unfrozen water remains at about
0°C despite the progressive cooling of the soil surface. Due to the
presence of this layer of ‘warmer’ water, ice crystallization leads to
an overcooling of soil water. The slow cooling rate and the low soil
water content result in the temperature of the overcooled unfrozen
water being as low as -5°C!
The intensity of cryogenic processes depends on temperature
factors (the degree, duration and depth of soil cooling below 0°C),
soil moisture, soil texture and mineralogical composition.
In clay soil, water is mainly bound to the surface of clay particles
as thin films of water. These films freeze at temperatures that are
much lower than 0°C. The most active freezing of film water is
observed at temperatures about –7°C.
Some film water remains unfrozen even at much lower temperatures.
This unfrozen water in the soil then migrates along the thermal
gradient from warm to cold, feeding the ice bodies. These growing
ice bodies provide the mechanism for cryogenic processes such as
frost heave, cryoturbation, cryogenic sorting and ice segregation.
If the temperature of the soil drops quickly to -40°C or lower, the
frozen soil mass shrinks and cracks in a manner similar to that which
occurs in any solid material.
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Weathering in cold climates
The above graph shows changes in temperature over time associated
with the freezing of a moist clay soil. In this example, ice crystallization
begins at -3.5°C. With the onset of crystallization, the temperature of
the soil rises due to the release of energy from the water (latent heat).
This illustrates that lowering air temperatures below 0°C may not be
sufficient for the development of cryogenic processes. (DK) [17]
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Physical weathering is driven by temperature fluctuations and the
growth of ice crystals in rock fissures. During the spring and summer,
cracks or fissures in rocks fill with water, which then freezes during
the onset of winter. As the volume of ice is greater than that of
water, the ice attempts to expand laterally causing the cracks in
rocks to be forced further apart. Eventually, the rock will shatter.
In very cold climates, another mechanism of physical weathering,
known as cryohydration, is widespread. Cryohydration involves the
splitting of individual mineral particles in the soil due to the freezing
and subsequent contraction of thin films of water that lie on the
surface of particles. This process results in the accumulation of finegrained (silt-size) particles in the soil.
A full description of weathering in soil is given in the section on ‘Soil
Forming Processes’.
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Ice and frost
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0
-2
-4
-6
t, °C
The graph above shows the relationship between temperature and the
percentage of unfrozen water content (Wu) in the pores of (1) heavy
clay loam, (2) light loam, (3) sandy clay loam, (4) sandy silt loam and (5)
loamy sand. Higher water contents lead to a slower freezing of the soil.
A sandy soil (5) will be completely frozen at -4°C (the graph reaches a
plateau) while a clay loam soil (1) will not be completely frozen at -6ºC
(plateau is not yet reached). (DK) [17]
Soil Atlas of the Northern Circumpolar Region | Soils in Northern Latitudes
Ice is the name given to the solid phase of water, a crystalline solid,
which can appear transparent or opaque bluish-white depending on
the presence of impurities such as air. Ice occurs when pure liquid
water is cooled to below 0°C (32°F) at standard atmospheric pressure.
Ice can form from a vapour with no intervening liquid phase, such as
in the formation of frost.
Frost is a broadly used term to describe the formation of ice crystals
that grow from a solid surface when the temperature of the object
is lower than the temperature (dew point) of the air. The size of frost
crystals depends on time and humidity.
In soil, frost can result in the formation of thin, long crystals known as needle
ice. Where the soil is very wet, solid blocks and wedges of ice can form.