Active layer
Encyclopedia
In environments containing permafrost
, the active layer is the top layer of soil
that thaws during the summer and freezes again during the autumn. In all climate
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 ambient temperature
is greatest. This means that, over many years, the influence of cooling in winter and heating in summer (in temperate climates) will decrease as depth increases.
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 (vegetation
).
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, while 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.
, 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 at Yakutsk
), 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 of sand
y or gravel
ly parent materials can be up to five times deeper than those made from loam
- or clay
-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 of plant
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 restricts tree
growth to specialised species such as Larix
. In areas of discontinuous permafrost, most conifers are able to grow easily.
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 for carbon dioxide
. This carbon dioxide and other greenhouse gases (chiefly methane
) 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 accelerate global warming
- some suggest the difference could become very significant especially if the carbon has been stored since before recent glacial maxima.
Permafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...
, the active layer is the top layer of soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
that thaws during the summer and freezes again during the autumn. In all climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
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 ambient temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
is greatest. This means that, over many years, the influence of cooling in winter and heating in summer (in temperate climates) will decrease as depth increases.
If the winter temperature is below the freezing point of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, 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 (vegetation
Vegetation
Vegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader...
).
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, while 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 depthThaw depth
In soil science, the thaw depth or thaw line is the level down to which the permafrost soil will normally thaw each summer in a given area....
, 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
Ellesmere Island
Ellesmere Island is part of the Qikiqtaaluk Region of the Canadian territory of Nunavut. Lying within the Canadian Arctic Archipelago, it is considered part of the Queen Elizabeth Islands, with Cape Columbia being the most northerly point of land in Canada...
), whilst if it is quite warm, it is much thicker (about 2.5 m at Yakutsk
Yakutsk
With a subarctic climate , Yakutsk is the coldest city, though not the coldest inhabited place, on Earth. Average monthly temperatures range from in July to in January. The coldest temperatures ever recorded on the planet outside Antarctica occurred in the basin of the Yana River to the northeast...
), and if the permafrost is discontinuous and soil begins thawing earlier, it can be still thicker (5 meters at Yellowknife
Yellowknife, Northwest Territories
Yellowknife is the capital and largest city of the Northwest Territories , Canada. It is located on the northern shore of Great Slave Lake, approximately south of the Arctic Circle, on the west side of Yellowknife Bay near the outlet of the Yellowknife River...
). The parent material of the soil is also important: active layers in soils made of sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
y or gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
ly parent materials can be up to five times deeper than those made from loam
Loam
Loam is soil composed of sand, silt, and clay in relatively even concentration . Loam soils generally contain more nutrients and humus than sandy soils, have better infiltration and drainage than silty soils, and are easier to till than clay soils...
- or clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
-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
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s of plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
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 restricts tree
Tree
A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
growth to specialised species such as Larix
Larch
Larches are conifers in the genus Larix, in the family Pinaceae. Growing from 15 to 50m tall, they are native to much of the cooler temperate northern hemisphere, on lowlands in the north and high on mountains further south...
. In areas of discontinuous permafrost, most conifers 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 formation in the active layer
CryoturbationCryoturbation
In gelisols , cryoturbation refers to the mixing of materials from various horizons of the soil right down to the bedrock due to freezing and thawing.Cryoturbation occurs to varying degrees in most gelisols...
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
Gelisols
Gelisols are an order in USDA soil taxonomy. They are soils of very cold climates which are defined as containing permafrost within two meters of the soil surface...
(permafrost soils) are very important as a sink for carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. This carbon dioxide and other greenhouse gases (chiefly methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
) 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 accelerate global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...
- some suggest the difference could become very significant especially if the carbon has been stored since before recent glacial maxima.