- Active layer
In environments containing
permafrost , the active layer is the top layer ofsoil that thaws during thesummer and freezes again during theautumn . In allclimate s, whether they contain permafrost or not, the temperature in the lower levels of the soil will remain more stable than that at the surface, where the influence of the ambienttemperature is greatest. This means that, over many years, the influence of cooling inwinter and heating in summer (in temperate climates) will decrease as depth increases. [citebook|title=Fundamentals of Geomorphology |author= Richard John Huggett|year=2003|publisher=Routledge|id=ISBN 0415241456|pages=237]If the winter temperature is below the freezing point of
water , a frost front will form in the soil. This "frost front" is the boundary between frozen and unfrozen soil, and with the coming of spring and summer, the soil is thawed, "always from the top down". If the heating during summer exceeds the cooling during winter, the soil will be completely thawed during the summer and there will be no permafrost. This occurs when the mean annual temperature is above 0°C (32°F), but also occurs when the mean annual temperature is slightly below 0°C on sites exposed to the sun with coarse-textured parent materials.When there is not sufficient heat to thaw the frozen soil completely, permafrost forms. The active layer in this environment consists of the top layers of soil which thaws during the summer, whilst the inactive layer refers to the soil below which is frozen year-round because the heat fails to penetrate. Liquid water cannot flow below the active layer, with the result that permafrost environments tend to be very poorly drained and boggy.
Thaw depth
The thickness of the active layer, known as the
thaw depth , is determined by how far the frost front is forced to retreat due to heating during the summer.Thus, the primary determinant of active layer thickness is the maximum temperature attained during the summer. If it is only a little above 0°C, the active layer can be very thin (only 10 cm on
Ellesmere Island ), whilst if it is quite warm, it is much thicker (about 2.5 m atYakutsk ), and if the permafrost is discontinuous and soil begins thawing earlier, it can be still thicker (5 meters at Yellowknife). The parent material of the soil is also important: active layers in soils made ofsand y orgravel ly parent materials can be up to five times deeper than those made fromloam - orclay -rich parent material. This is because the coarser material allows for much greater conductivity of heat down into the soil.This is important because
root s ofplant s cannot penetrate beyond the active layer and are restricted by its thickness. Thus, in a continuous permafrost environment plants must have shallow roots, which restrictstree growth to specialised species such as "Larix". In areas of discontinuous permafrost, mostconifer s are able to grow easily.Pereletok
Because of variation in summer temperatures from year to year, the amount of heating of the active layer also varies - hence the depth of thaw is not constant. Soil that will thaw only during a hotter-than-average summer is known as Pereletok (a Russian word meaning "old ice". It can also be referred to as an Intergelisol.
oil formation in the active layer
Cryoturbation is the dominant force operating in the active layer, and tends to make it generally uniform in composition throughout. However, variation in the composition of soils due to differences in parent rock are very marked in permafrost regions due to the low rate of weathering in the very cold climate.The slow rate of decomposition of organic material means
Gelisols (permafrost soils) are very important as a sink forcarbon dioxide . This carbon dioxide and othergreenhouse gases (chieflymethane ) forms from the very slow decomposition of the excess organic matter that remains in most Gelisols and is mixed down into the pereletok layer during relatively hot summers and below that layer during warmer periods about 5000 to 6000 years ago. This storage of carbon means thawing of permafrost may accelerateglobal warming - some suggest the difference could become very significant especially if the carbon has been stored since "before" recent glacial maxima.See also
References
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