A low, often oval, frost heave occurring in polar and subpolar climates
Palsas are peat mounds with a permanently frozen peat and mineral soil core. They are a typical phenomenon in the polar and subpolar zone of discontinuous permafrost. One of their characteristics is having steep slopes that rise above the mire surface. This leads to the accumulation of large amounts of snow around them. The summits of the palsas are free of snow even in winter, because the wind carries the snow and deposits on the slopes and elsewhere on the flat mire surface. Palsas can be up to in diameter and can reach a height of .
Permafrost is found on palsa mires only in the palsas themselves, and its formation is based on the physical properties of peat. Dry peat is a good insulator, but wet peat conducts heat better, and frozen peat is even better at conducting heat. This means that cold can penetrate deep into the peat layers, and that heat can easily flow from deeper wet layers in winter, whereas the dry peat on the palsa surface insulates the frozen core and prevents it from thawing in the summer. This means that palsas can survive in a climate where the mean annual temperature is just below the freezing point.
A lithalsa is a palsa without peat cover. They exist in a smaller range than palsas, commonly occurring in oceanic climate regimes. However both palsas and lithalsas are relatively small compared to pingos, typically less than .
Palsa development.
Palsas may be initiated in areas of a moor or bog where the winter freezing front penetrates relatively faster than surrounding areas, perhaps due to an unusually thin cover of snow. The lack of thermal insulation provided by thick snow permits much deeper freezing in winter. This ice may then last through the summer with a persistent 'bump' of up to several cm due to frost heave. The elevated surface of a palsa will tend also to have thinner snow cover, allowing greater winter cooling, while in summer the surface material (especially if organic) will dry out and provide thermal insulation. Thus the interior temperature is consistently lower than that of adjacent ground. This contributes to the formation of an ice lens which grows by drawing up surrounding water. The expansion of the ice upon freezing exerts pressure on the surrounding soil, further forcing water out of its pore spaces which then accumulates on and increases the volume of the growing ice lens. A positive feedback loop develops. Changes in surface moisture and vegetation will then be such as to preserve the newly formed permafrost.
The overlying soil layer is gradually lifted up by frost heaving. In cross-section, the ice cores of a palsa show layering, which is caused by the successive winter freezing intervals. The pressing out of water from the pores is not crucial, however, since the boggy soil is water-saturated and thus always provides enough water for ice core growth.
Many scientists agree that the development of a palsa is cyclic where growth continues until a convex form of the palsa is reached. When this occurs an increasing pressure in the uppermost layer of peat will cause cracks in the peat layer which will result in the sliding of the peat layer toward the sides of the palsa. As this layer of peat generates an insulating effect the regression of the layer will thereby expose the permafrost in the palsa and initiate melting. In this case, the melting of the palsa is a normal part of the cyclic development and, it will be possible for new embryonic palsa forms to develop in the same area. However, the studies done on palsa forms has primarily been observing dome palsas in the northern regions. These study areas lie within the core area for palsa occurrences and therefore are the cyclic development applicable only to dome palsas within the core area.
Palsa plateaus often lack the convex form which causes cracks in the peat layers and the decay of dome palsas. But in palsa plateaus, frost expansion which causes swelling will with time create an uneven surface and increase the possibility for water accumulation on the surface and cause local regression and melting. This process, which causes melting likewise the cracking of the peat layer in dome palsas, is a normal part in the life span of palsa plateaus but are not a part of a cyclic evolvement.
Palsas appear to go through a developmental cycle that eventually leads to thawing and collapse. Open cracks that commonly accompany palsa growth and the water that tends to accumulate around palsas, probably as a result of their weight depressing the adjacent bog surface, are important factors in this process. The fact that palsas in various stages of growth and decay occur together shows that their collapse is not necessarily indicative of climatic change. All that is usually left after a palsa collapses is a depression surrounded by a rim.
Morphology.
One specific type of mire at which palsa structures appear is called a palsa mire. But, sometimes the nature type is described as palsa bogs, however, they both refer to a peaty wetland where palsa mounds occur. In palsa mires, palsas which are in different stages of development can appear due to the cyclic development of the structure. Therefore, the collapsed form of the palsas are common in these areas which can be seen as rounded ponds, open peat surfaces or low circular rim ridges.
The individual palsa is described as a mound or a larger elevation in peatland with a core of permanently frozen peat and/or mineral soil with an uppermost active layer of peat. The landform occurs in areas with discontinuous permafrost. The core of palsas stays frozen permanently, including summertime, as the peat layer creates an insulating effect. Mostly palsas have an oval or elongated form but different shapes of palsas have been described. In some places (Laivadalen and Keinovuopio in northern Sweden), palsa complexes which consist of several dome-shaped palsas have been found. At other places (Seitajaure in northern Sweden), another palsa structure is described. Here several palsa-plateaus have been found which have flatter surfaces and steep edges.
Palsa forms include mounds, plateaus and ridges of different sizes. Palsas in Iceland have been described as hump-shaped, dike-shaped, plateau-shaped, ring-shaped, and shield-shaped. Those in Norway have been referred to as palsa plateaus, esker palsas, string palsas, conical or dome-shaped palsas, and palsa complexes.
Widths are commonly , and lengths . However, lengths of up to have been reported for esker-like palsa ridges running parallel to the gradient of a bog. Heights range from less than up to , but can reach about at a maximum above the surrounding area. Large forms tend to be considerably less conical than small ones. In places, palsas combine to form complexes several hundred meters in extent. The permafrost core contains ice lenses no thicker than , though locally lenses up to almost thick have been described.
During the cyclic development, the palsa goes through several stages at which the morphology differs. In the initial aggrading stage of development, the palsas have smooth surfaces with no cracks in the peat layer and no visible signs of erosion can be seen. They are often small and dome-shaped and often referred to as embryo palsas. In this stage ice layers are created which are commonly found in the frozen peat core. It has been suggested that these ice layers are created by ice segregation but, it is most certainly buoyancy that is the reason for the formation of the ice layers. Buoyant rise of the core occurs which freezes when the permafrost reaches the area and creates the ice layers. In the stable, mature phase, the surface has risen further to a level at which the snow cover during winter is thinned by the wind which in turn makes it possible for deeper freezing. In the mature stage, the frozen core has reached beyond the peat layer into the underlying silty sediments and during summer thawing of the core occurs but not to an extent where the core thaws completely. The thawing can sometimes create water filled ponds adjacent to the palsa and in some cases, cracks in the peat layer along these ponds can be present in the stable stage. However, these cracks are small in size and no visible signs of block erosion are seen during the sable stage. During the degrading stage, however, the palsas have large cracks up to several meters which divide the peat layer into blocks and so-called block erosion occurs. Adjacent to palsas in the degrading stage often several individual ponds are found, due to thawing of the frozen core. Wind erosion often affect the peat layer to such a degree that it decreases in thickness with sometimes several decimeters. When palsa plateaus are in the degrading stage several ponds on the flat plateau-surface can be seen which often have neighbouring block erosion. When block erosion occurs the mineral soil is often exposed along the cracks, especially when the peat layer is thin.
Geographic distribution.
Palsas are typical forms of the discontinuous permafrost zone regions and are therefore found in Subarctic regions of northern Canada and Alaska, Siberia, northern Fennoscandia and Iceland. They are almost exclusively associated with the presence of peat and commonly occur in areas where the winters are long and the snow cover tends to be thin. In some places palsas extend into underlying permafrost; in others they rest on an unfrozen substratum.
In the southern hemisphere palsa remains from the last glacial maximum have been identified on the Argentine side of Isla Grande de Tierra del Fuego just north of Cami Lake. Remainders of Ice-Age palsas are to be found also in Hochmooren of Central Europe, such as Hohen Venn in the German-Belgian border area.
Effects of climate change.
Effect on palsa forms due to change in climatic conditions.
Erosion of palsa forms and the receding of the permafrost in the core of the palsa does not directly indicate a change in climatic conditions. As the palsas have a cyclic development the thawing of the core is a normal part of the palsa development. However, change in climatic conditions does affect palsa forms. The palsa forms that lay in the outskirt of the occurrence area are more dependent on climatic conditions for existence than the palsa forms near the core of the occurrence area. A study on palsa forms was done in 1998 at Dovrefjell, in southern Norway. At the time of observation, the mean annual temperature lied just under in the area. These areas are certainly sensitive to changes in temperature; just a small temperature rise can have a great effect on the lasting existence of palsas in the specific region. Measurements from meteorological stations in the area show that the mean annual temperature rose 0.8 °C between the time periods of 1901–1930 and 1961–1990. Since the start of the warming trend in the 1930s, entire palsa bogs and large palsa plateaus have completely melted in the Dovrefjell area. Palsa bogs' sensitivity to changes in temperature makes them a good climate indicator. The study in the Dovrefjell area concluded that if palsas are used as climate indicators it is essential to separate large changes in the distribution of permafrost from smaller changes. Smaller changes are caused by shorter climatic variations which only last a few years. Small dome palsas, which also can be called embryo palsas, can develop as a result of smaller variations in climatic conditions such as a few following cold winters. As these small palsas disappear after just a few years, they fail to establish as permanent formations. This phenomenon has been observed in the Dovrefjell in the last decades and is caused by a larger change in the climatic condition where the temperature has risen to a level at which the palsas cannot fully initiate their cyclic development. This is a consequence of climate change with the warming trend which has been observed in the Dovrefjell area. In this area, the climate has not been cold enough for new palsa forms to establish during the whole of the 20th century.
However, some uncertainties of how the local conditions affect the formation of palsa forms and especially the hydrology of palsa mires still exist. Additionally, more active-layer monitoring and its correlation to local weather conditions is needed to