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Land Management & Natural
Hazards Unit |
SOIL |
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Soil
Atlas of |
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Northern latitudes
Europe is part of the circumpolar segment of the global soil coverage. This part of the atlas describes the pattern of soil distribution around the Arctic, a pole of snow and ice. The harsh continental climate with a low amount of solar radiation, intense cold during long winter and limited heating during the short summer provide a negative heat balance on these lands. The cold affects the surrounding landscape and leads to the formation of permanently frozen ground known as permafrost (soil having an annual temperature within the upper one meter layer of below 0oC). Cryosol is the most characteristic soil
type of the northern latitudes and is often found in association with
Histosols and Gleysols. Cryosols form from coarse-textured deposits and from
fresh alluvial or aeolian parent material associated with Podzols, Planosols and/or
Cambisols. Cryosols have a number of specific
soil-forming processes that distinguish them from other soil. Most of these
processes are driven by a soil water regime that migrates along a thermal
gradient from warm to cold sites. This move has different directions due to
the seasonal freeze-thaw cycle and may cause cryoturbation, frost heave,
cryogenic sorting, thermal cracking and ice segregation. Repeated freezing and thawing of water in
the soil results in lifting of coarse rock fragments, cryoturbation (i.e. is
the churning of minerals by hard frost) and mechanical crashing (physical
weathering) of rocks. During freeze-back (the freezing portion of the cycle),
freezing fronts move both from the soil surface downward and from the
permafrost table upward. As this happens, moisture is removed from the
unfrozen soil material between the two fronts (frost desiccation).
Desiccation is responsible for the development of blocky structures in these
soils; the combination of cryoturbation and desiccation causes the granular
structure of many fine-textured Cryosols. The cryostatic pressure that
develops as the freezing fronts merge results in a higher bulk density of the
soil. Cryoturbation (frost churning) mixes the
soil materials and results in irregular or broken soil horizons, involutions,
organic intrusions, organic matter occurrence in the subsoil, oriented rock
fragments, silt-enriched layers, silt caps and oriented micro-fabrics. Two
models have been suggested to explain the cryoturbation process:
Frost heave is caused by an expansion of
soil volume due to freezing either because of the change in volume that takes
place when water is converted to ice or because ice buildup in the subsoil
causes cracks to form in the soil. Re-freezing causes coarse fragments in the
soil to be heaved and sorted resulting in oriented features in the soil and
micro-topography of patterned ground. These include circles (including earth
hammocks), nets, polygons, stripes and steps. Thermal cracking is formed when frozen
materials contract under rapid cooling. The resulting cracks are typically
several centimetres wide. They might become filled in with water or sand
later to form ice or sand wedges. Since old thermal cracks are zones of
weakness, cracking recurs at the same place. Ice segregation refers to the accumulation
of ice in cavities and hollows in the soil mass and manifests itself in a
variety of phenomena. Features include ice lenses, vein ice, ice crystals and
some types of ground ice. The characteristic platy and blocky
macro-structures of Cryosols result from vein ice development. Cryosols can have Gleyic properties that
develop from prolonged saturation with water during the thaw period. Cryosols
might also manifest features of podzolization if derived from coarse-textural
deposits. In the cold deserts, Cryosols might have alkaline properties and
salts or show signs of reddening (Rubefaction). |
While the subsoil of Cryosols remains permanently frozen (the permafrost layer), the upper portion thaws in summer. The maximum thaw depth of the seasonally frozen layer represents the depth of the active layer. The latter is highly dynamic and depends on climate and environmental changes. The active layer in Cryosols can extends up to 80 cm below the soil surface and depends on the physical environment of the soil and the soil texture, soil moisture regime and thickness of organic topsoil. When the temperature drops, freeze-back occurs from the frost table upwards and from the soil surface downwards. The soil material between these freezing fronts comes under cryostatic pressure and, as a result, unfrozen materials are displaced and soil horizons are contorted and broken. This causes the characteristic cryogenic structure and cryoturbated soil horizons. Weak leaching and translocation of materials occur in permafrost soil giving rise to leached horizons. However, the evidence of the transformation of soil material is often partially or completely destroyed by cryoturbation, which mixes the soil materials in the active layer. European soil : a global perspectiveCryosols with a high silt content are thixotropic, such that viscosity decreases when stirred or shaken, to become liquified gel but returns to the hardened state upon standing. The active soil layer in Cryosols supports biological activity and protects the underlying permafrost. Soil texture, moisture regime, thickness of organic surface layer, vegetation cover, aspect and latitude are among the factors that control the thickness of the active layer. Salt crusts are common on soil surfaces on the high arctic islands of Canada and Russia. These salt crusts develop during dry periods in the summer because of increased evaporation from the soil surface. The pH of Cryosols varies greatly and depends on the composition of the parent material: Cryosols developed in calcareous parent material have a higher soil-pH than soils in non-calcareous material. The similarity of the soil-pH to that of the parent material is also caused, in part, by cryoturbation, which mixes soil materials not only between horizons but also with the parent materials.
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