Soils of the Cryosolic order occur throughout northern Canada and are the dominant soil type throughout most of the territories (northern Yukon, Northwest Territories, and Nunavut). Cryosols are also common in the regions around the Hudson Bay and Hudson Strait, particularly in northeastern Manitoba, northwestern Ontario, and the northernmost part of Quebec (the Ungava peninsula in Nunavik). These regions are characterized by long, cold winters and short, cool summers. As a consequence, the mean annual soil temperature is at or below 0°C, resulting in permafrost conditions, where the ground remains frozen for two or more consecutive years. The frequent freeze-thaw cycles associated with these cold environments contribute not only to the presence of permafrost near the soil surface but also to a suite of soil-forming processes known as cryoturbation. Cryoturbation refers to soil movement that arises from frost action, and is sometimes also referred to as “frost churning”.
There are a few different commonly accepted models to explain how cryoturbation occurs; just a few examples are listed here. In the cryostatic model, a freezing front advancing from both above (cold air) and below (permafrost) can exert pressure on the soil of the active layer. In the convective cell equilibrium model, heave-subsidence processes occurring at both the top and bottom of the active layer move soil upward and outward. Both the formation and subsequent melting of ice lenses within the active layer can give rise to the characteristic broken or irregular soil horizons commonly associated with cryoturbation. Where changes in temperature and moisture contribute to cracking in the soil, ice and/or sand wedges can develop.
Generally, cryoturbation is most common in soils with finer textures (fine sand to silt) and wet conditions, but has been observed under other environmental conditions as well. The relative importance of a given cryoturbation mechanism or model in developing a particular Cryosolic profile will depend on the environmental conditions at that particular location.
Although many of the same soil-forming processes that occur in other orders also take place in Cryosols (for example, the reduction-oxidation processes associated with Gleysols or the build-up of organic material associated with the Organic order), the near-surface permafrost contributes to a particular suite of soil properties that include, but are not limited to, horizons that have been affected by cryoturbation. The diagnostic horizons associated with the Cryosolic order are represented by the suffixes ‘y’ (for horizons with evidence of cryoturbation) and ‘z’ (for frozen material, i.e., permafrost). These suffixes can be used alone with a major horizon (O, A, B, C) or together with another suffix to reflect a combination of soil-forming processes.
In those regions of Canada where the active layer is thicker (such as the discontinuous permafrost zone), Cryosols may be found in association with other soil orders. Soils of the Gleysolic and Organic orders may be found in wetter, low-lying areas of the landscape where the active layer is greater than 1 m thick. Similarly, Brunisolic and Regosolic soils may be found on well-drained knolls and slopes, particularly south-facing slopes. Relict Cryosols (i.e., soils with evidence of cryoturbation but that no longer meet the permafrost criteria for the Cryosolic order) can be found in some southern environments, such as the boreal forest, reflecting a historically colder soil-forming environment. These features are generally not well represented in the Canadian System of Soil Classification.
The information presented above reflects some of the extensive research done by cryopedologists such as Charles Tarnocai (Agriculture and Agri-Food Canada) and James Bockheim (University of Wisconsin).
Cryosolic Great Groups
The great groups of the Cryosolic order are defined by two main criteria: a) the presence or absence of obvious cryoturbation, and b) the nature of the parent material (mineral or organic). The specific criteria for each group are shown in the table below (adapted from the Canadian System of Soil Classification, 3rd Ed.).
|
Turbic
|
Static
|
Organic
|
|
| Cryoturbation evident |
Yes
|
No
|
No
|
| Parent material |
Mineral
|
Mineral
|
Organic
|
| Depth to permafrost |
≤2 m
|
≤1 m
|
≤1 m
|
Organic
Soils in the Organic great group have formed in organic parent material (i.e., material with >17% organic carbon by weight. While similar to soils of the Organic order in terms of the diagnostic thickness of the organic layers, they must also have permafrost within 1 m of the surface. They generally do not have evidence of cryoturbation, although there may be patterned ground, such as peat polygons.
Turbic
The Turbic great group is distinguished from the other two great groups primarily by the marked evidence of cryoturbation. Within the soil pit, this will appear as irregular or broken horizons and evidence of mixing of the horizons, such as involutions, wedge-shaped horizons, and/or oriented stones. At the soil surface, this may also appear as patterned ground, including sorted and non-sorted circles, polygons, stripes, or earth hummocks (in finer materials). Given the instability associated with actively turbic sites, surface vegetation may be minimal or completely absent, with parent material evident right at the soil surface.
Static
Static Cryosols are mineral soils without evidence of cryoturbation. They are distinguished from similar soils of other orders (such as Brunisols) by the presence of permafrost within 1 m of the surface. They commonly occur in association with the other two great groups but tend to be found in portions of the landscape with very coarse or recently deposited parent material and/or low moisture levels inhibit the development of turbic features. For example, at Truelove Lowland on Devon Island, Static Cryosols tend to be found on the raised beach crests whereas Turbic Cryosols are more prevalent in the mid- to lower-slope positions, which may be separated by less than a metre of elevation difference.
Cryosolic Subgroups
The subgroups of the mineral Cryosolic great groups are based on a) base saturation as indicated by pH, b) presence of a Bm, Bt, Bgy and/or Cgy - or absence of a B horizon, c) presence of a thick organic horizon near the surface, or d) certain combinations of a) through c).
The subgroups of the Organic great group are based on similar criterion to those of the Organic order, namely a) the degree of decomposition, and b) the depth to mineral contact (exception: Glacic subgroup).
| Subgroup |
Great Group
|
||
| Turbic | Static | Organic | |
| Gleysolic |
X
|
X
|
Not applicable
|
| Histic Eutric |
X
|
X
|
Not applicable
|
| Histic Dystric |
X
|
X
|
Not applicable
|
|
Histic Regosolic
|
X
|
X
|
Not applicable
|
| Luvisolic |
Not applicable
|
X
|
Not applicable
|
| Brunisolic Eutric |
X
|
X
|
Not applicable
|
| Brunisolic Dystric |
X
|
X
|
Not applicable
|
| Orthic Eutric |
X
|
X
|
Not applicable
|
| Orthic Dystric |
X
|
X
|
Not applicable
|
| Regosolic |
X
|
X
|
Not applicable
|
| Glacic |
Not applicable
|
Not applicable
|
X
|
| Terric Fibric |
Not applicable
|
Not applicable
|
X
|
| Terric Mesic |
Not applicable
|
Not applicable
|
X
|
| Terric Humic |
Not applicable
|
Not applicable
|
X
|
| Fibric |
Not applicable
|
Not applicable
|
X
|
| Mesic |
Not applicable
|
Not applicable
|
X
|
| Humic |
Not applicable
|
Not applicable
|
X
|
Gleysolic
These are the poorly drained Cryosols that have developed under saturated or reducing conditions due to the depth of the water table and/or their position in the landscape, resulting in low chromas or mottles in the mineral horizons (Static GG: Bg &/or Cg, Turbic GG: Bgy &/or Cgy)
Histic Eutric
These soils have a Bm horizon that is less than 10 cm thick (or a Bmy in the case of the Turbic GG) with a pH ≥ 5.5. They are found primarily in better-drained positions, although there may be a gleyed layer above the permafrost table. These soils also have an organic (Om) horizon >15 cm thick within 1 m of the surface, reflecting substantial organic matter inputs.
Histic Dystric
Similar to the Histic Eutric, but more acidic: the pH of the Bm horizon is <5.5.
Histic Regosolic
Similar to the Histic subgroups above, but lacking a B horizon.
Luvisolic
Found only in the Static GG, these soils have evidence of clay translocation in the form of an eluvial (Ae) horizon, and an illuvial (Bt or Bty) horizon ≥10 cm thick.
Brunisolic Eutric
These soils are very similar to the Histic Eutric subgroup, but lack the thick organic horizon. In addition, their Bm horizon is at least 10 cm thick and not affected by cryoturbation. The pH and drainage characteristics are the same.
Brunisolic Dystric
Similar to the Brunisolic Eutric soils, but the pH of the Bm horizon is <5.5.
Orthic Eutric
These soils are similar to the Brunisolic Eutric soils (lack a thick organic layer and pH≥ 5.5), but the Bm (or Bmy) horizon is less than 10 cm thick.
Orthic Dystric
Similar to the Orthic Eutric, but the pH of the Bm horizon is <5.5.
Regosolic
These are the young and/or unstable Cryosols, lacking B horizons
The following Subgroups are found only in the Organic GG and are differentiated primilary by the degree of decomposition in the control section. For the Terric Organic SG, the control section is just above the mineral contact. For the remaining SG, the control section is the organic material below 40 cm depth.
Glacic
These soils have a thick (>30 cm) layer of ground ice (Wz) in the upper 1 m of the profile. The ice layer is at least 95% ice by volume (as opposed to mostly soil with ice in the pore spaces).
Terric Fibric
Like the subgroups of the Organic order, these soils are distinguished by having a mineral contact within 1 m of the soil surface and fibric organic matter (Ofz) above the contact.
Terric Mesic
These soils have a mineral contact within 1 m of the soil surface and mesic organic matter (Omz) above the contact.
Terric Humic
These soils have a mineral contact within 1 m of the soil surface and humic organic matter (Ohz) above the contact.
Fibric
These are similar in degree of decomposition to the Terric Fibric, but because these soils lack a mineral contact, the control section used to determine their subgroup classification is just below a depth of 40 cm.
Mesic
Soils similar in degree of decomposition to the Terric Mesic, but lacking a mineral contact.
Humic
Soils similar in degree of decomposition to the Terric Humic, but lacking a mineral contact.