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Gelisols
Gelisols
Summary:
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Vegetation: lichens, moss, liverwort, sedges, grass
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Climate: pergelic
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Soil moisture regime: variety of soil moisture regimes
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Major soil property: accumulation of organic matter, special features
formed by cryoturbation
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Diagnostic horizons: -
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Epipedon: histic
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Major processes: cryoturbation
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Characteristics: soils that contain within 200 cm of the ground surface
permafrost (permanently frozen ground)
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Gelisols -
Environmental Conditions
Climate: Gelisols develop in climatic regions where temperatures
continuously are at or below 0° C (e.g. alpine, polar regions) - pergelic
temperature regime. They occur in arid regions and areas with effective
precipitation. Permafrost (i.e., permanently frozen soil) is a characteristic
environmental factor for the development of Gelisols. The distribution of
permafrost comprise two zones: (i) continuous permafrost: the zone at the
highest latitudes and elevations where permafrost is ubiquitous; the southern
boundary corresponds to the -7 °C isotherm. (ii) discontinuous (sporadic)
permafrost: the zone in which permafrost occurs only in some materials; the
southern boundary corresponds to the 0 to -2 °C isotherm.
Vegetation: Cold climate inhibits the growth of many species; only
species adapted to harsh cold environmental conditions can survive, for
example, lichens, sphagnum moss, liverwort, sedges, grass, Picea Betula,
Salix.
Relief: There is no limitation in relief for the formation of Gelisols.
Parent Material: Gelisols can form in any parent material. Often
they form in glacial drift material.
Time: At very low temperatures (< 0 to -70° C) pedogenic
processes are slowed down, i.e., soil development is very slow. Many soils
in cold regions are very old, e.g. in Antarctica millions of years.
Gelisols - Processes
Definition of Permafrost (permanently frozen soil): A condition existing
below the ground surface, irrespective of texture, water content, or geologic
character, in which the temperature of the material remained below 0 °C
continuously for two or more years. The soil above the permafrost that
thaws in the summer is referred to as the 'active layer'.
Cryopedogenesis is the sum of all sub-processes
occurring in cryogenic
soils, including compaction (desiccation), displacement (alignment, rotation,
sorting, inclusions), and pore formation.
Cryoturbation (frost churning), which is mixing of soil due to
freezing and thawing, results in the disruption of horizons, displacemetn
of soil material, the incorporation of organic matter into lower horizons,
and the orientation of stones in the soil profile. Cryoturbation in the soil
profile is manifested by irregular and broken horizons and textural bands,
involutions, organic matter accumulation on the permafrost table, oriented
stones, silt caps and accumulations, and deformed soil material associated
with movements due to ice- and sand-wedge growth.
At 0 °C, the increase in volume with the conversion from water to
ice is 9 percent. When the moisture contained in rocks freezes, and the accompanying
internal pressures are sufficiently great to exceed the strength of the
rock, the rock ruptures (thermal cracking). Freezing, associated
with an expansion of soil water, and thawing, associated with a contraction
or collapse of soil layers, result in a new terrain called hilly thermokarst.
Frost cracking of ground into polygons results from shrinkage of
the ground during cold dry winters. Water from the active layer in summer
seeps into cracks and freezes, starting the growth of vertical ice wedges.
With the approach of winter, refreezing of the moist soil may be by simultaneous,
slow upward extension of cementing ice above the permafrost table and
downward extension of surface freezing ground. Subsoil between these two
approaching freezing fronts develops a massive condition from centuries
of this seasonal compaction.
Patterned ground formation is a process which results in special
features such as circles of stones, nets, polygons, steps, or stripes.During summer periods the upper few centimeters or several decimeters
of a pedon thaws. On slopes (> 1 % gradient) the upper soil layer, which
is highly saturated with meltwater flows above the upper surface of the permafrost,
called permafrost table. This process is called solifluction. Within
solifluction layers stones are transported downhills to depression areas.At low temperatures, particles of snow are as hard as grains of bedrock
and can 'sandblast' ventifacts (wind erosion).
Pedogenic processes in very cold environments such as weathering,
transformations and translocations of mineral and organic materials are
slow. Because decomposition is retarded in cold climate and organic
matter is accumulated and histic epipedons are formed.
Gelisols - Properties
The dark black epipedon in many Gelisols is classified as histic
, which is formed by low decomposition rates.
Freezing and thawing in the zone above the permafrost table forms features
such as unsorted and sorted circles of stones, nets, polygons, steps,
stripes, mounds, pingos, peat rings, and beaded drainage patterns. Freezing
and thawing forms platy and vesicular structures in surface mineral horizons,
and blocky, prismatic, and massive structures in subsoil. Ice lenses may
form close to the permafrost table.
Because pedogenic processes are retarded in cold regions the soil landscapes
with Gelisols are fragile. It takes very long time periods to wipe out
the impact of disturbances, for example, produced by human activity such
as extracting geologic materials or digging of soil pits.
Gelisols - Classification
Gelisols occur in arctic regions such as Antarctica, Russia, Canada, Alaska.
An estimated 13.4 % (18 million km2)
of soils of the planet are occupied by permafrost. The basic requirement
to form Gelisols is:
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the presence of permafrost within 100 cm of the soil surface; or
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gelic materials within 100 cm of the soil surface and permafrost within
200 cm of the soil surface.
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Gelic materials are mineral or organic materials that have evidence of
cryoturbation and/or ice segregation in the active layer (seasonal thaw
layer) and/or upper part of the permafrost.
Soil horizon symbols (Gelisols):
jj: cryoturbation
ff: dry permafrost
Wfm: glacic horizon (> 75 % ground ice in a layer >= 30 cm thick)
There are three different suborders of Gelisols:
Histels: Histels are organic soils similar to Histosols exept that
they have permafrost within 2 meters below ground surface. They have 80
% or more organic materials from the soil surface to a depth of 50 cm or
to a glacic layer or densic, lithic, or paralithic contact, whichever is
shallowest. These soils occur predominantely in Subarctic and Low Arctic
regions of continuous or widespread permafrost. Less than one-third of
the active layer (the soil between the ground surface and a permafrost
table) or an ice layer which is at least 30-cm thick has been cryoturbated.
Turbels: Turbels are soils that show marked influence of cryoturbation
(more than one-third of the active-layer portion of the pedon) such as
irregular, broken, or distorted horizon boundaries and involutions and
areas with patterned ground. They commonly contain tongues of mineral and
organic horizons, organic and mineral intrusions and oriented rock fragments.
Organic matter is accumulated on top of the permafrost and ice wedges are
a common features in Turbels. These soils occur primarily in the zone of
continuous permafrost.
Orthels: Orthels are soils that show little or no cryoturbation
(less than one-third of the pedon). Patterned ground (except for polygons)
generally is lacking. These soils occur primarily within the zone of discontinuous
permafrost, in alpine areas where precipitation is greater than 1400 mm
per year.
The decomposition stage of organic material (fiber in the OM) distinguishs
Gelisols on the great group and subgroup level. 'Fibric' (e.g. Fibristels),
'Hemic' (e.g. Hemistels), and 'Sapric' (e.g. Sapristels) organic material
is considered to distinguis Gelisols at the great group level. 'Sphagnic'
indicates the presence of sphagnum moss which influences soil development.
In 'Humic' Gelisols a mollic, umbric, or histic epipedon is present (e.g.
Humiturbels), in 'Umbric' Gelisols there is an umbric epipedon (e.g. Umbriorthels),
and in 'Mollic' Gelisols show a mollic epipedon (e.g. Molliorthels).
'Glacic' (> 75 % ground ice in a layer >= 30 cm thick) is used at
great group and subgroup level to classify Gelisols (e.g. Glacic Folistels,
Glacic Aquaturbels).
The presence of calcium sulfate defines 'Gypsic' Gelisols (e.g. Gypsic
Anhyturbels), soluble salts define 'Salic' Gelisols (e.g. Salic Anhyturbels),
carbonates defines 'Calcic' Gelisols (e.g. Calcic Anhyturbels).
Gelisols with an argillic diagnostic horizon (e.g. Argiorthels) or a spodic
horizon (Spodic Psammiorthels) are also considered in the classification
of Gelisols.
Gelisols developed in sandy parent material are designated by the term
'Psammentic' (e.g. Psammentic Aquorthels, Psammiturbels).
'Sulfuric' Gelisols show a mineral or organic horizon that has a pH <
3.5, inhibits growth of plant roots, and has yellow mottles of jarosite
(e.g. Sulfuric Aquaturbels).
There are two general types of permafrost: (i) dry permafrost, which contains
insufficient interstitial water to cement the soil matrix, and (ii) wet
frozen or ice-cemented permafrost, which contains sufficient moisture to
cement the soil matrix. Gelisols formed in dry permafrost regions are classified
as 'Anhy' (e.g. Anhyturbels, Anhyorthels). The denotion 'Foli' is used
to classify Gelisols which are saturated with water only a few days each
year (e.g. Folistels). Gelisols, which are saturated seasonally are classified
as 'Aquic' (e.g. Aquistatels, Aquic Umbriorthels).
Gelisols formed in volcanic material (e.g. volcanic glass) which does
not meet the criteria of the Andisol order are considered by the formative
element 'vitrandic' (e.g. Vitrandic Molliorthels) or 'andic' if the fine-earth
fraction exhibits a bulk density of 1.0 g/cm3
or less (e.g. Andic Andic Molliorthels).
Soil depth distinguishs between 'Lithic' - shallow (e.g. Lithic Anhyorthels)
and 'Cumulic' - accumulated (e.g. Cumulic Umbriorthels) Gelisols. Gelisols,
which have a mineral layer 30 cm or more thick that has its upper boundary
within the control section below the surface tier are classified as 'Terric'
(e.g. Terric Sapristels).
Gelisols - Distinguishing Characteristics
In the past, Soil Taxonomy has identified Gelisols as pergelic subgroups
of Entisols, Inceptisols, Histosols, Mollisols, and Spodosols.
Gelisols and Histosols show high organic matter contents, whereas Gelisols
are limited to cold climate.
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