Thermokarst is a land surface characterised by very irregular surfaces of marshy hollows and small hummocks formed as ice-rich permafrost thaws, that occurs in Arctic areas, and on a smaller scale in mountainous areas such as the Himalayas and the Swiss Alps. These pitted surfaces resemble those formed by solution in some karst areas of limestone, which is how they came to have karst attached to their name without the presence of any limestone. Small domes that form on the surface due to frost heaving with the onset of winter are only temporary features. They then collapse with the arrival of next summer's thaw and leave a small surface depression. Some ice lenses grow and form larger surface hummocks, which last 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 contribute to uneven surfaces. These are included within the general label of thermokarst. The formation of permafrost thaw lakes due to warming climate is a positive feedback loop, as methane and carbon dioxide are released as permafrost thaws, contributing to further climate warming. 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, refers to a body of freshwater, usually shallow, that is formed in a depression formed by thawing ice-rich permafrost. 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. 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. Thermokarst lakes tend to form and disappear in a cyclical manner, resulting in a predictable life cycle. These lakes are typically found in arctic and subarctic lowlands, including the western Canadian Arctic, the Alaskan coastal plain, interior Yukon Territory and the alluvial lowlands of northern Eurasia and Siberia. 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 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. The disturbance 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.
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. Through discontinuous permafrost, it is when thaw occurs in palsas or in lithalsas. 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.
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”. It should be specified that “drunken trees” 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. 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. Additionally, thermal abrasion of thermokarst lake edges can expand the lake size, as well as lake bottom subsidence. Additionally, oriented morphology of lakes can take on shapes such as “elliptical, egg-shaped, tri- angular, rectangular, clam-shaped, or D-shaped”, and commonly occur in terrain with sandy sediments. Throughout the literature on orientation and morphology of thermokarst lakes, polemic scholastic discussions pertaining to how lake shape is developed are commonplace. However, Grosse et al. publication Thermokarst lakes, drainage, and drained basins summarizes endogenous and exogenous elements that are key factors in orientation including: redistribution of littoral shelves by wind creating insulation, arrangement of polygonal ice-wedges producing thawing, and erosion from fluvial channels causing inhomogeneous sediments. Clearly there is a multitude of reasonings to explain the shape of lakes, not just centred around wind movement.
Drainage
Before complete drainage, lake edges recede through retrogressive thaw slumping 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 may be caused by local permafrost degradation and erosion. Lakes stop growing once drainage 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.
