Thermokarst

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Permafrost thaw ponds in Hudson Bay, Canada in 2008. Permafrost thaw ponds in Hudson Bay Canada near Greenland.jpg
Permafrost thaw ponds in Hudson Bay, Canada in 2008.

Thermokarst is a type of terrain characterised by very irregular surfaces of marshy hollows and small hummocks formed as ice-rich permafrost thaws. The land surface type occurs in Arctic areas, and on a smaller scale in mountainous areas such as the Himalayas and the Swiss Alps.

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These pitted surfaces resemble clusters of small lakes formed by dissolution of limestone in some karst areas, which is how they came to have " karst " attached to their name, even though no limestone is actually present. Small domes that form on the surface due to frost heaving with the onset of winter are only temporary features. They collapse during the following summer thaw, leaving a small surface depression. Some ice lenses grow and form larger surface hummocks ("pingos") which can last for many years, and sometimes become covered with grasses and sedges, until they begin to thaw. These domed surfaces eventually collapse – either annually or after longer periods – and form depressions which become part of the uneven terrains included under the general category of thermokarst.

The formation of permafrost thaw lakes due to warming climate is a positive feedback loop, as methane, nitrous oxide and carbon dioxide are released as permafrost thaws, contributing to further climate warming. [1] [2] The Batagaika crater in Siberia is an example of a large thermokarst depression.

Thermokarst lakes

A thermokarst lake, also called a thaw lake, tundra lake, thaw depression, or tundra pond, [3] refers to a body of freshwater, usually shallow, that is formed in a depression formed by thawing ice-rich permafrost. [4] A key indicator of thermokarst lakes is the occurrence of excess ground ice as well as having an ice content with greater than 30% by volume. [5] Thermokarst lakes tend to form and disappear in a cyclical manner, resulting in a predictable life cycle (see "life cycle" below). Continued thawing of the permafrost substrate can lead to the drainage and eventual disappearance of thermokarst lakes, leaving them, in such cases, a geomorphologically temporary phenomenon, formed in response to a warming climate. [6]

These lakes are typically found in arctic and subarctic lowlands, including the western Canadian Arctic [7] (e.g. Banks Island, Victoria island), the Alaskan coastal plain, [8] [9] interior Yukon Territory [10] and the alluvial lowlands of northern Eurasia and Siberia. [11] [12] [13] [14] The presence of thaw lakes in a region results in a thermal disturbance as the water warms the ground.

The depth of permafrost below a lake will generally be shallower and if the lake is of sufficient depth, a talik is present. The general morphology (shape, depth, circumference) is variable, with some thaw lakes oriented, meaning they are generally elongated in a specific direction. Though their formation mechanism has not been definitively proven, it is believed to be related to the prevailing winds or storms. [15] The disturbance (of either kind) leads to overall warming and melting of ground ice, after which surface subsidence occurs allowing for water infiltration of either surface water or melted ground ice. [5]

Teshekpuk Lake on the Alaska North Slope within the National Petroleum Reserve-Alaska is the largest thermokarst lake in the world. [16]

Lake life cycle

Initiation

The initiation of a thaw lake begins with the degradation of ice-rich permafrost. The natural inception of thermokarst lakes can be demarcated into two separate processes; whether in continuous or discontinuous permafrost. In continuous permafrost, water accumulates when ice veins and polygonal ground are present. [17] Through discontinuous permafrost, it is when thaw occurs in palsas (frozen peat cores) or in lithalsas (mineral core mounds). [18] Permafrost degradation is typically linked to a surface disturbance, either natural or artificial, in combination with site-specific factors, such as permafrost ice-content, ground temperature, etc. [19]

Development/expansion

Development of thaw lakes tends to be slow at first, but once the average lake bottom temperature exceeds 0 °C the lake ceases freezing to the bottom and thaw becomes continuous. The lake grows as ice thaws, which may result in the slumping of shorelines or submergence of vegetation, which is why thaw lakes in the boreal forest tend to be surrounded by “drunken trees”. [19] It should be specified that “drunken trees” (also known as drunken forests) occur within Yedoma regimes. This feature is not present throughout all thermokarst regions. Upon expansion in this stage, thermokarst lakes often take on an elongated shape with ordered alignment in the long axis. [15]

If lakes form in an area of ice-rich permafrost, coalescence of several smaller lakes may occur, producing a larger body of water, magnifying the thermal disturbance. Development may be further facilitated by lateral bank erosion. [15] Additionally, thermal abrasion of thermokarst lake edges can expand the lake size, as well as lake bottom subsidence. [20]

Oriented morphology of lakes can take on shapes such as “elliptical, egg-shaped, triangular, rectangular, clam-shaped, or D-shaped”, [5] and commonly occur in terrain with sandy sediments. [5] Polemic scholastic discussions pertaining to development of lakes’ shapes are commonplace throughout the literature on orientation and morphology of thermokarst lakes. However, there are clearly a multitude of reasons beyond wind movement only, that contribute to the shape of lakes. Grosse et al. (2013) [5] summarize endogenous and exogenous elements that are key factors in orientation including:

Drainage

Before complete drainage, lake edges recede through retrogressive thaw slumps (RTS) and subaerial debris flows. Actual drainage may be triggered by fluvial erosion or expansion of adjacent basins at inland locations. In coastal areas, drainage may be due to coastal retreat leading to thermal abrasion or erosion due to wave action. More gradual drainage (partial or complete) may be caused by local permafrost degradation and erosion. [5] Lakes stop growing once drainage is initiated, and eventually depressions are filled by sediments, aquatic plants or peat. Another option for the fate of a drained thaw lake is that the active layer surrounding the lake deepens to below water level once ground ice is exhausted, allowing for a residual lake to remain. [19]

See more photos at Wikimedia Commons - Thermokarst.

See also

Related Research Articles

<span class="mw-page-title-main">Permafrost</span> Soil frozen for a duration of at least two years

Permafrost is soil or underwater sediment which continuously remains below 0 °C (32 °F) for two years or more: the oldest permafrost had been continuously frozen for around 700,000 years. While the shallowest permafrost has a vertical extent of below a meter, the deepest is greater than 1,500 m (4,900 ft). Similarly, the area of individual permafrost zones may be limited to narrow mountain summits or extend across vast Arctic regions. The ground beneath glaciers and ice sheets is not usually defined as permafrost, so on land, permafrost is generally located beneath a so-called active layer of soil which freezes and thaws depending on the season.

Landforms are categorized by characteristic physical attributes such as their creating process, shape, elevation, slope, orientation, rock exposure, and soil type.

<span class="mw-page-title-main">Herschel Island</span> Island in Yukon, Canada

Herschel Island is an island in the Beaufort Sea, which lies 5 km (3.1 mi) off the coast of Yukon in Canada, of which it is administratively a part. It is Yukon's only offshore island.

<span class="mw-page-title-main">Kobuk River</span> River in Alaska, United States

The Kobuk River' is a river located in the Arctic region of northwestern Alaska in the United States. It is approximately 280 miles (451 km) long. Draining a basin with an area of 12,300 square miles (32,000 km2), the Kobuk River is among the largest rivers in northwest Alaska with widths of up to 1,500 feet and flow at a speed of 3–5 miles per hour in its lower and middle reaches. The average elevation for the Kobuk River Basin is 1,300 feet (400 m) above sea level, ranging from near sea level to 11,400 feet. Topography includes low, rolling mountains, plains and lowlands, moderately high rugged mountainous land, and some gently sloped plateaus and highlands. The river contains an exceptional population of sheefish, a large predatory whitefish within the salmon family, found throughout the Arctic that spawns in the river's upper reaches during the autumn. A portion of the vast Western Arctic Caribou Herd utilize the Kobuk river valley as winter range.

<span class="mw-page-title-main">Bolshoy Lyakhovsky Island</span> Island in Lyakhovsky Islands, Russia

Bolshoy Lyakhovsky Island, or Great Lyakhovsky, is the largest of the Lyakhovsky Islands belonging to the New Siberian Islands archipelago between the Laptev Sea and the East Siberian Sea in northern Russia. It has an area of 5,156.6 km2 (1,991.0 sq mi), and a maximum altitude of 311 m (1,020 ft).

<span class="mw-page-title-main">Pingo</span> Mound of earth-covered ice

Pingos are intrapermafrost ice-cored hills, 3–70 m (10–230 ft) high and 30–1,000 m (98–3,281 ft) in diameter. They are typically conical in shape and grow and persist only in permafrost environments, such as the Arctic and subarctic. A pingo is a periglacial landform, which is defined as a non-glacial landform or process linked to colder climates. It is estimated that there are more than 11,000 pingos on Earth. The Tuktoyaktuk peninsula area has the greatest concentration of pingos in the world with a total of 1,350 pingos. There is currently remarkably limited data on pingos.

<span class="mw-page-title-main">Teshekpuk Lake</span> Lake on the coast of Alaska

Teshekpuk Lake is the largest lake in Arctic Alaska, at 22 miles (35 km) width on the Alaska North Slope within the National Petroleum Reserve-Alaska, South of Pitt Point, 12 miles (19 km) east of Harrison Bay, 80 miles (130 km) east of Point Barrow. The Teshekpuk Lake region is considered one of the most productive, diverse, and sensitive wetland ecosystems in the entire Arctic, habitat to a variety of arctic wildlife, including the resident Teshekpuk Lake caribou herd of 64,000 animals, large numbers of shorebirds and migratory waterfowl, for whom it is an essential part of the East Asian–Australasian Flyway site network.

Aufeis is a sheet-like mass of layered ice that forms from successive flows of ground or river water during freezing temperatures. This form of ice is also called overflow, icings, or the Russian term, naled. The term "Aufeis" was first used in 1859 by Alexander von Middendorff following his observations of the phenomenon in northern Siberia.

Yedoma is an organic-rich Pleistocene-age permafrost with ice content of 50–90% by volume. Yedoma are abundant in the cold regions of eastern Siberia, such as northern Yakutia, as well as in Alaska and the Yukon.

<span class="mw-page-title-main">Drunken trees</span> Stand of trees displaced from their normal vertical alignment

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A periglacial lake is a lake bordering a glacier, usually found along the fringes of large ice sheets.

<span class="mw-page-title-main">Arctic methane emissions</span> Release of methane from seas and soils in permafrost regions of the Arctic

Arctic methane release is the release of methane from seas and soils in permafrost regions of the Arctic. While it is a long-term natural process, methane release is exacerbated by global warming. This results in a positive feedback cycle, as methane is itself a powerful greenhouse gas.

<span class="mw-page-title-main">Sergey Zimov</span>

Sergey Aphanasievich Zimov is a Russian geophysicist who specialises in arctic and subarctic ecology. He is the Director of Northeast Scientific Station, a senior research fellow of the Pacific Institute for Geography, and one of the founders of Pleistocene Park. He is best known for his work in advocating the theory that human overhunting of large herbivores during the Pleistocene caused Siberia's grassland-steppe ecosystem to disappear and for raising awareness as to the important roles permafrost and thermokarst lakes play in the global carbon cycle.

Scalloped topography is common in the mid-latitudes of Mars, between 45° and 60° north and south. It is particularly prominent in the region of Utopia Planitia, in the northern hemisphere, and in the region of Peneus and Amphitrites Paterae in the southern hemisphere. Such topography consists of shallow, rimless depressions with scalloped edges, commonly referred to as "scalloped depressions" or simply "scallops". Scalloped depressions can be isolated or clustered and sometimes seem to coalesce. A typical scalloped depression displays a gentle equator-facing slope and a steeper pole-facing scarp. This topographic asymmetry is probably due to differences in insolation. Scalloped depressions are believed to form from the removal of subsurface material, possibly interstitial ice, by sublimation. This process may still be happening at present. This topography may be of great importance for future colonization of Mars because it may point to deposits of pure ice.

<span class="mw-page-title-main">Frost boil</span> Small circular mounds of fresh soil material formed by frost action and cryoturbation

A frost boil, also known as mud boils, a stony earth circles, frost scars, or mud circles, are small circular mounds of fresh soil material formed by frost action and cryoturbation. They are found typically found in periglacial or alpine environments where permafrost is present, and may damage roads and other man-made structures. They are typically 1 to 3 metres in diameter.

<span class="mw-page-title-main">Permafrost carbon cycle</span> Sub-cycle of the larger global carbon cycle

The permafrost carbon cycle or Arctic carbon cycle is a sub-cycle of the larger global carbon cycle. Permafrost is defined as subsurface material that remains below 0o C for at least two consecutive years. Because permafrost soils remain frozen for long periods of time, they store large amounts of carbon and other nutrients within their frozen framework during that time. Permafrost represents a large carbon reservoir, one which was often neglected in the initial research determining global terrestrial carbon reservoirs. Since the start of 2000s, however, far more attention has been paid to the subject, with an enormous growth both in general attention and in the scientific research output.

<span class="mw-page-title-main">Periglaciation</span>

Periglaciation describes geomorphic processes that result from seasonal thawing and freezing, very often in areas of permafrost. The meltwater may refreeze in ice wedges and other structures. "Periglacial" originally suggested an environment located on the margin of past glaciers. However, freeze and thaw cycles influence landscapes also outside areas of past glaciation. Therefore, periglacial environments are anywhere when freezing and thawing modify the landscape in a significant manner.

<span class="mw-page-title-main">Batagaika crater</span> Thermokarst crater in Siberia, Russia

The Batagaika crater is a thermokarst depression in the Chersky Range area. The biggest permafrost crater in the world, it administratively belongs to the Sakha Republic, Russia, and is in its Verkhoyansky District.

In geomorphology fluvio-thermal erosion is the combined mechanical and thermal erosion of an unfrozen river or stream against ice-rich soils and sediments. The erosional process includes the thawing of ice sediments by a strong water flow and once the surface is unfrozen, mechanical erosion occurs only if hydraulic forces are powerful enough to incise the riverbank material. This kind of erosion sometimes causes the banks to collapse into the river, and when this occurs collapses are commonly controlled by ice wedges. Rivers where this process has been observed include the Lena, the Colville River delta, and the Yukon River.

Retrogressive thaw slumps (RTS), are a type of landslide that occur in the terrestrial Arctic's permafrost region of the circumpolar Northern Hemisphere when an ice-rich section thaws. RTSs develop quickly and can extend across several hectares modifying Arctic coastlines and permafrost terrain. They are the most active and dynamic feature of thermokarst—the collapse of the land surface as ground ice melts. They are thermokarst slope failures due to abrupt thawing of ice-rich permafrost or glaciated terrains. These horseshoe-shaped landslides contribute to the thawing of hectares of permafrost annually and are considered to be one of the most active and dynamic features of thermokarst—the "processes and landforms that involve collapse of the land surface as a result of the melting of ground ice." They are found in permafrost or glaciated regions of the Northern Hemisphere—the Tibetan Plateau, Siberia, from the Himalayas to northern Greenland, and in northern Canada's Northwest Territories (NWT), the Yukon Territories, Nunavut, and Nunavik and in the American state of Alaska. The largest RTS in the world is in Siberia—the Batagaika Crater, also called a "megaslump", is one-kilometre-long and 100 metres (330 ft) deep and it grows a 100 feet (30 m) annually. The land began to sink, and the Batagaika Crater began to form in the 1960s, following clear-cutting of a section of forested area.

References

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