2.1.1 Relict ice

Here, relict ice is considered to be larger bodies or layers of ice preserved in permafrost under overburden by persistent cold-climate conditions. Relict ice may consist of either (1) buried glacier ice (French and Harry, 1988, 1990; Lorrain and Demeur, 1985; St-Onge and McMartin, 1995) or (2) intrasedimental ice formed by segregation, injection, or a combination thereof (Mackay and Dallimore, 1992; Rampton, 1988a). Buried glacier ice in Canada is predominantly found within large hummocky moraine belts and glaciofluvial deposits (Coulombe et al., 2019; Dyke and Savelle, 2000; French and Harry, 1990; Kokelj et al., 2017; Lakeman and England, 2012; Murton et al., 2005) and may consist of englacial or basal ice preserved by an overburden of ablation till or outwash (Evans, 2003). Intrasedimental ice has been identified in some regions of northern Canada (Dallimore and Wolfe, 1988; French and Harry, 1990; Fritz et al., 2011; Mackay and Dallimore, 1992; Murton, 2005; Pollard, 2000). This ice is considered to form by segregation and/or intrusion at the interface between layers of fine-grained and coarse-grained deposits during epigenetic permafrost growth (Mackay, 1971). The underlying unfrozen stratum supplies the water for ice growth. Isotopic evidence indicates that massive intrasedimental ice is commonly of glacial meltwater origin (Mackay and Dallimore, 1992). Thick ice growth may be facilitated by high pore water pressures that develop during freezing of saturated sands and gravels (pore water expulsion) or by glacial meltwater driven toward aggrading permafrost near an ice sheet margin (Konrad, 1990; Mackay and Dallimore, 1992; Rampton, 1988a). Debate remains over the origin (buried glacier ice vs. intrasedimental ice) of some ice-cored terrain in northern Canada due to the similar morphological appearance, associated surficial materials, and distribution near margins of the late-Wisconsinan Laurentide Ice Sheet (Evans, 2003). The relict ice model does not account for the localized hydraulic and stratigraphic conditions that may form intrasedimental ice, so the results should be considered representative of relict buried glacier ice, though intrasedimental ice may form locally within the same glacigenic deposits (Murton et al., 2005).

2.1.2 Segregated ice

Segregated ice consists of discrete ice lenses or layers that form by migration of unfrozen pore water towards a freezing front (Taber, 1929). This segregated ice consists of relatively thin ice lenses and layers that form various cryostructures and is thus differentiated from larger intrasedimental ice bodies formed by segregation and/or intrusion. Segregated ice is widely distributed in fine-grained material in Canada, whereas large intrasedimental ice bodies formed in spatially and temporally restricted proglacial settings during recession of the Laurentide Ice Sheet (Rampton, 1988a) or smaller ice caps (Robinson and Pollard, 1998).

Segregated ice growth results in frost heaving, a displacement of the ground surface due to the volumetric increase in the soil as ice forms (Miller, 1972). Segregated ice forms during epigenetic (downward) permafrost growth and due to annual downward water migration from the active layer into permafrost (Cheng, 1983; Mackay, 1983). Gradual ice enrichment near the top of permafrost and a rising permafrost table due to climatic cooling, organic accumulation, or sedimentation in alluvial, eolian, or colluvial sediments forms aggradational ice, resulting in an upper ice-rich “intermediate” layer found commonly in permafrost regions (French and Shur, 2010). Slow epigenetic freezing may form the segregated ice expressed as lithalsas, palsas, and peat plateaus (Allard et al., 1996; Calmels et al., 2008a; Laberge and Payette, 1995; Wolfe et al., 2014). Examples of segregated ice enrichment include the ice visible in exposures above thaw unconformities (Burn, 1988; Kokelj and Burn, 2003), in near-surface sediments of drained lake basins (Mackay and Burn, 2002a; Morse et al., 2009; O'Neill and Burn, 2012), in aggraded alluvial settings (Kokelj and Burn, 2005; Morse et al., 2009), and in loess sequences (Fortier et al., 2018; Fraser and Burn, 1997; Kotler and Burn, 2000). Segregated ice may be expressed as distinct cryofacies in polygenetic permafrost, where epigenetic permafrost growth is followed by surface sediment or organic matter accumulation and subsequent syngenetic permafrost development (e.g., French and Shur, 2010; Gilbert et al., 2018).

2.1.3 Wedge ice

Wedge ice develops when low winter temperatures cause the ground to contract under stress and crack (Lachenbruch, 1962). Meltwater later infills the cracks and freezes in permafrost, forming thin ice veins. Cracking in subsequent years occurs preferentially at existing ice veins because the tensile strength of the ice is lower than that of the surrounding frozen sediment (Lachenbruch, 1962). Repeated cracking, water infilling, and refreezing causes the growth of an ice wedge over time. Ice-wedge growth is thus controlled by winter temperature conditions and the time available for ice accumulation (Hamilton et al., 1983; Mackay and Burn, 2002b; Opel et al., 2018). Wedge ice may comprise a significant volume of the upper ground in permafrost areas (Kanevskiy et al., 2013; Pollard and French, 1980; Ulrich et al., 2014), and the partial thaw of ice wedges can lead to ground subsidence that spurs marked ecological changes (Fraser et al., 2018; Jorgenson et al., 2006; Liljedahl et al., 2016).

4.1.1 Relict ice

Areas with high modelled relict ice abundance correspond well with the distribution of large thaw slumps, which typically develop in areas underlain by thick layers of relict ice (Kokelj et al., 2017), helping validate the model in northwest Canada (Fig. 5). The model reproduces relict ice observed directly or interpreted from thaw slump mapping on Banks Island, Victoria Island, the western Arctic mainland, the central Canadian Shield, and Ellesmere Island (Fig. 5b; see Fig. S3 for specific locations mentioned throughout the text). Areas of high ice abundance are associated with hummocky till and moraine deposits that have remained in tundra and continuous permafrost during the Holocene. Field observations of relict ice from the discontinuous permafrost zone are lacking, as is indirect evidence in the form of large thaw slumps (Fig. 5b). The model also depicts relict ice in medium and lower abundance in other glacigenic terrain of the continuous permafrost zone that was not inundated during the Holocene. It is difficult to evaluate results in many of these areas due to the lack of field evidence. Nonetheless, it seems reasonable that relict ice may remain preserved in other till and glaciofluvial deposits.

Figure 5(a) Modelled abundance of relict ice and (b) abundance of large (>1 ha) thaw slumps in northwest Canada (Kokelj et al., 2017; Segal et al., 2016), and field observations of relict ice from the literature (Table S3).

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4.1.2 Segregated ice

Areas with high modelled segregated ice abundance occur in raised marine and in glacial lake sediments and correspond to observations of widespread shallow thaw lakes, lithalsas, palsas, and peat plateaus (Fig. 6). High segregated ice abundance is modelled in Old Crow Flats in northern Yukon where there are about 2700 shallow thermokarst lakes with an average depth of <2 m (Turner et al., 2014). In epigenetic permafrost, segregated and wedge ice is concentrated in the upper sediments, which explains the shallow thaw lake depths (Matthews et al., 1990; Roy-Leveillée and Burn, 2015, 2017). High segregated ice abundance is modelled in the Hudson Bay Lowlands and is evident in the form of widespread palsas and peat plateaus in the uplifted fine-grained marine sediments (Kuhry, 2008; Payette, 2004). High abundance is also modelled around Umiujaq on eastern Hudson Bay, where both palsas and lithalsas are common (Allard and Seguin, 1987; Calmels et al., 2008b). Southwestern Baffin Island also includes high segregated ice abundance where shallow (<2 m) thaw lakes and drained lake basins are widespread on the Great Plain of the Koukdjuak (Martini and Morrison, 2014).

Figure 6Modelled abundance of segregated ice. Old Crow Basin (1) is characterized by widespread excess ice in fine-grained lacustrine sediments and an abundance of shallow thermokarst lakes. Within the Hudson Bay Lowlands (2), areas with high segregated ice abundance include extensive raised peat plateaus. Lithalsas are widespread in raised marine sediments at Umiujaq (3). The Great Plain of the Koukdjuak (4) and southern Boothia Peninsula (5) also include fine-grained uplifted marine sediments, and shallow thaw lakes are abundant in both regions.

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High segregated ice abundance modelled on small parts of the Fosheim Peninsula on Ellesmere Island correspond to areas covered by fine-grained, ice-rich deposits (Pollard, 1991). Massive intrasedimental ice may underlie these deposits and is considered to have formed by segregation during permafrost aggradation into sands and silt overlying unfrozen coarse strata with highly pressurized groundwater (Pollard, 2000; Robinson and Pollard, 1998). These specific formation conditions following marine emergence are not considered in either the relict or segregated ice model, but the general near-surface ice conditions are nonetheless represented by the high modelled segregated ice abundance. Medium segregated ice abundance is modelled over much of the western Arctic mainland and Banks and Victoria islands in till derived from sedimentary bedrock. These areas also include abundant shallow thaw lakes (Burn, 2002; Dallimore et al., 2000) and thaw slumps (e.g., Lantz and Kokelj, 2008). Further south in the discontinuous permafrost zone, small thaw slumps and active layer detachment slides occur in the Mackenzie Valley where segregated ice abundance is largely modelled as low or medium (e.g., Couture and Riopel, 2008).

4.1.3 Wedge ice

The modelled distribution of wedge ice generally corresponds to field and remote-sensing observations of ice wedges and polygonal terrain. The size of ice wedges and the density of polygonal terrain decreases southward across the tundra–forest transition in the western Arctic (Kokelj et al., 2014), and the wedge ice model reproduces this general pattern as an increase from negligible to high wedge ice abundance (Fig. 7). Areas of modelled negligible wedge ice abundance in forest–tundra generally correspond to small ice wedges observed only in organic soils. Low wedge ice abundance corresponds to small to moderately sized ice wedges observed in both mineral and organic soils in shrub tundra (Kokelj et al., 2014). Areas of medium ice-wedge abundance correspond to medium to large ice wedges observed further north in dwarf-shrub tundra (Fig. 7). Pollard and French (1980) estimated that wedge ice on Richards Island constitutes about 12 % of the upper 4.5 m of permafrost, where the modelled abundance is medium to high (Fig. 7a).

Figure 7Comparison of (a) modelled wedge ice abundance to (b) wedge widths and the density of polygonal terrain in the western Canadian Arctic (modified from Kokelj et al., 2014, Fig. 6).

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High wedge ice abundance is modelled over large areas of Banks and Victoria islands (Fig. 4). Evidence of abundant large ice wedges is available from imagery and from field investigations on Banks Island. Ice wedges may exceed 3 m in width and 10 m in depth and comprise up to 15 % of the volume of upper ground (French, 1974). The formation of large ice wedges in this environment occurs due to the persistence of an Arctic climate for millennia. In addition, wedge growth may be facilitated where it occurs in relict ice, which has a high thermal contraction coefficient, promoting wide and deep thermal contraction cracking (Lachenbruch, 1962). A recent image analysis of Banks Island highlighted the widespread degradation of large ice wedges and the resulting broadscale increase in ponding in flatter terrain (Fraser et al., 2018).

Recent estimates on Fosheim Peninsula indicate that ice-wedge polygons cover about 50 % of the terrain and that wedge ice occupies between 1.4 % and 5.9 % volume of the upper 5.9 m of permafrost (Bernard-Grand'Maison and Pollard, 2018). In this region, wedge ice abundance is modelled mostly as low, with some small areas of medium abundance (Fig. 4). The modelled wedge ice abundance and field- or imagery-derived wedge volume estimates are thus generally consistent for Banks Island, Richards Island, and Fosheim Peninsula.

4.2.1 Relict ice

Both representations generally depict abundant ice on the western Arctic mainland and Banks and Victoria islands, but the model output is more spatially defined and more accurately reflects current knowledge of the distribution of relict ice (Figs. 2, 8a). For example, no relict ice is modelled on southeastern Victoria Island, where marine inundation is considered to have melted relict ice (Figs. 2, 8d). This interpretation is supported by the lack of large thaw slumps mapped in the region (Fig. 5b; Kokelj et al., 2017). The PMC indicates abundant relict ice over most of the Canadian Arctic Archipelago islands northeast of Banks and Victoria islands (Fig. 8a, d). In contrast, the model indicates that relict ice is absent on most of these islands, as the surficial materials are regolith or colluvium and are thus unlikely to include large preserved ice bodies. In addition, many of the islands were partially or fully inundated following deglaciation. On the Great Plain of the Koukdjuak, western Baffin Island, the PMC indicates abundant relict ice. However, the region was also marine inundated, and the shallow depths and consistent shape of the lake basins there indicate thaw of uniformly distributed segregated ice restricted to upper sediments rather than of thicker relict ice bodies (Martini and Morrison, 2014). In the Peel Plateau region, areas of high and medium modelled relict ice abundance occur where widespread exposures have been observed directly and mapped using remotely sensed imagery (Kokelj et al., 2015; Lacelle et al., 2015). In contrast, the PMC indicates that relict ice is absent or sparse in this region (Fig. 8a, d). The model indicates an absence of relict ice in Old Crow Flats, whereas the PMC indicates that it is abundant (Fig. 8d). The region includes abundant ice wedges, a thin layer of near-surface segregated ice, and resulting shallow thermokarst lakes (Labrecque et al., 2009; Roy-Leveillée and Burn, 2017), but evidence of relict ice is lacking. The past submergence of the area by glacial Lake Old Crow and associated deposition of thick glaciolacustrine sediments precludes the preservation of relict ice, so the modelled output is more appropriate. In the northern Mackenzie Valley region, the relict ice abundance is higher on the PMC than the model output (Fig. 8d). Low, negligible, and no relict ice is indicated by the model due to the presence of different till units (hummocky, blanket, veneer) and reductions in modelled abundance due to Holocene tree line advance in the region. The large physiographic unit with abundant ice on the PMC extends south to areas with little evidence in the form of large thaw slumps (Fig. 5b). In these examples the modelled distribution of relict ground ice is in better agreement with available empirical data and more spatially discrete than the portrayal on the PMC. Further empirical investigations are required to assess the model outputs in some areas, such as on Brodeur Peninsula on northwest Baffin Island and on the central Canadian Shield, where the modelled abundance is higher than on the PMC (Fig. 8d).

4.2.2 Segregated ice

The distribution of segregated ice is also broadly similar in the PMC and model output, with relatively high abundance portrayed over much of the western Arctic mainland and islands (Figs. 3, 8b). However, some key differences are evident (Fig. 8e). For example, ice content is considered low-medium in the Hudson Bay Lowlands on the PMC, whereas the model output indicates high abundance in the extensive fine-grained marine sediments (Fig. 8e). Smith and Burgess (2004) similarly mapped this region as having high thaw sensitivity due to high ground ice content. Given the abundance of peat plateaus and palsas in this region and the evident landscape change that has occurred due to recent ground ice melt (Payette, 2004), the high modelled segregated ice abundance is more appropriate. The PMC indicates a higher abundance of segregated ice than the model in some areas on eastern and high Arctic islands (Fig. 8e). However, many of these areas are covered by weathered regolith or bedrock, which are not highly frost susceptible and thus likely contain little excess ice. The modelled abundance is higher than the PMC in some areas mapped as fine-grained colluvium, notably on parts of Axel Heiberg and Melville islands. High segregated ice abundance is indicated on the northeast Tuktoyaktuk coastlands on the PMC, whereas the model indicates a general absence (Fig. 8e). The surficial materials in this area are largely eolian sand and sandy outwash that contain little segregated ice (Rampton, 1988b). Some small areas with frost-susceptible lacustrine deposits are also present (Rampton, 1988b) but not mapped at the scale of the surficial geology dataset used in the model. The PMC indicates high segregated ice abundance in the Mackenzie Valley area south of the tree line, where the modelled abundance is reduced to represent periodic thaw and loss of ice from the upper permafrost (Burn, 1988; Shur et al., 2005). This modelled pattern requires further investigation. The model indicates high or medium segregated ice abundance in many areas of the central Canadian Shield that include frost-susceptible sediments, whereas the PMC indicates low-medium or low ice content in the encompassing physiographic regions (Fig. 8e). The difference map in Fig. 8e highlights how the surficial geology base map leads to a more detailed depiction of segregated ground ice that accounts for the heterogeneity in frost susceptibility of surficial materials. The model depiction is an improvement in several regions, but further evaluation using field datasets is required in some areas to better assess the outputs.

4.2.3 Wedge ice

Broad patterns of wedge ice abundance are similar in the model output and the PMC, with areas of high abundance concentrated in the western Arctic (Figs. 4, 8c). However, the PMC indicates more wedge ice on many of the islands of the central and northern Canadian Arctic Archipelago (Fig. 8f). The modelled abundance on these islands is generally low or negligible because the substrate is dominantly coarse-grained and thin regolith, which does not favour ice-wedge development. Small areas of medium and high abundance are modelled near some coasts in fine-grained till and marine deposits and in fine-grained colluvium in areas with sedimentary bedrock. The lower modelled abundance on most of the islands is a more reasonable depiction, particularly considering the lack of polygonal terrain visible in imagery compared to Banks Island, where large ice wedges are widely distributed and the modelled abundance is high (Fraser et al., 2018). Abundant ice wedges are indicated in the PMC on southwest Baffin Island on the Great Plain of the Koukdjuak (Fig. 8c). In contrast, our model results indicate only negligible to low wedge ice abundance in the area (Fig. 4), partly because the plain has only been terrestrially exposed for ca. 4 kyr. The modelled abundance better represents field conditions, as only small incipient ice wedges occur on the plain (Martini and Morrison, 2014). In the northern Mackenzie Valley and Mackenzie Delta regions, modelled wedge ice abundance is lower than indicated on the PMC due to the modelled decrease in wedge ice across the tundra–forest transition (Figs. 4, 8f). As indicated in Sect. 4.1.3, this pattern reproduces the observed southward decrease in polygonal terrain density and wedge width on the western Arctic mainland (Kokelj et al., 2014). Modelled wedge ice abundance is high or medium in the Peel Plateau region but sparse or absent on the PMC. In this case, field observations suggest that the PMC better represents wedge ice conditions, as the distribution of polygonal ground on Peel Plateau is limited to small peatlands that occupy a small proportion of the overall landscape (Steve Kokelj, personal communication, 2018). Section 5.1.2. discusses why the model likely over-represents wedge ice in the region.

These comparisons highlight the limitations of representing ground ice conditions in large physiographic units that may include a range of surficial materials and landscape histories. Overall, the model outputs offer an improved representation of known ground ice conditions by accounting for specific surficial materials and considering the effects of broadscale environmental processes. The apparent inaccuracies in the PMC in many areas necessarily impact the output of other broadscale landscape assessments, such as mapping of thermokarst landscapes and permafrost hazard potential, that use the ground ice distribution as input parameters (e.g., Nelson et al., 2001; Olefeldt et al., 2016).

5.1.1 Input data validity

One of the main challenges of representing complex phenomena over large areas is the precision and accuracy of the source information. The surficial material dataset used in the models, though more spatially detailed than physiographic units used in previous mapping (Brown et al., 1997; Heginbottom et al., 1995) lacks precision and accuracy in some areas. The mapping by Fulton (1995) synthesized maps that were typically 1 : 50 000 to 1 : 250 000 in scale. The map units for the national representation were compiled at 1 : 1 000 000 in scale and then photographically reduced to 1 : 5 000 000 in scale (Fulton, 1995). This reduction caused the removal of some small map units and a loss of detail in the surficial geology dataset (Fulton, 1995). In addition, many areas of Canada and particularly the Arctic remained unmapped at the time of compilation. In these areas, Fulton (1995) relied on available literature and assessments by experts to delineate surficial units. These deficiencies caused inaccuracy in the modelled results in some areas. For example, lithalsas are abundant in glaciolacustrine sediments along the northern shore of Great Slave Lake near Yellowknife, Northwest Territories (Wolfe et al., 2014). Segregated ice is modelled as either absent or negligible in much of this area because surficial materials are mapped as bedrock and till veneer. More recent mapping has properly identified the widespread glaciolacustrine sediments that host the many ice-cored features in the region (Stevens et al., 2017; Wolfe et al., 2014). Therefore, the model output could be improved by including updated surficial geology mapping. These data are becoming available in a consistent digital format through new mapping and conversions of existing maps by the Geological Survey of Canada (Deblonde et al., 2018).

The generalized paleogeographic datasets, although fundamental to the model, are inherently limited and subject to scrutiny regarding the timing and spatial extent of the last glaciation, deglacial limits, lake and sea levels, and biome distributions. For example, the paleovegetation dataset of Dyke et al. (2004) has several limitations that affect model results. The biome distributions are based on a relatively large dataset of pollen sites and plant and terrestrial mammal macrofossils across northern North America (n=4659), but these observations are most limited in modern herb, shrub, and forest tundra environments. Therefore, the interpretation of biomes and their limits are approximations, particularly in northern regions. Second, the biome distributions reflect climatic factors such as growing season length and the mean temperature of the warmest month, which are not direct analogues of ground thermal conditions. Climatic and vegetation conditions may vary significantly within each of the defined biomes, affecting ground thermal conditions controlling, for example, wedge ice development or the preservation of relict ice.

5.1.2 Conceptual validity

Conceptual validity refers to the justifiability of underlying theories and assumptions in the model that represent the system behaviour (Rykiel, 1996). For each model, the cause–effect relations and assumptions are explained in the methods section and Supplement. Necessary assumptions in the expert system do not fully capture the full complexity of conditions known to influence ground ice growth and decay. For example, the relict ice model does not deal with ice melt over time in tundra environments of the continuous permafrost zone resulting from erosional processes. Melt of relict ice may occur irrespective of climatic amelioration due to colluvial, fluvial, or coastal erosion processes, which are controlled by topography, site-scale conditions, and energy and mass transfers that are not represented in the models.

For the wedge ice model, complex relations among air temperature, snow and vegetation conditions, and ground temperature were overly simplified under the broad assumption that ice-wedge cracking frequency increases as air temperature and snow cover decrease from forest to herb tundra (e.g., Kokelj et al., 2014). Regional deviations in air temperature and snow accumulation controlled by synoptic weather patterns and orography were not considered, which may influence wedge ice abundance. For example, the model indicates areas of high wedge ice abundance on Peel Plateau, Northwest Territories, in fine-grained glacigenic sediments that have remained in herb and shrub tundra biomes for much of the Holocene. However, polygonal terrain is sparse in the region. Ice-wedge development may be limited on Peel Plateau because winter ground temperatures in the uplands are relatively high, owing to strong winter air temperature inversions and relatively thick snow cover compared to shrub tundra north of the latitudinal tree line (O'Neill et al., 2015), and these specific regional conditions are not captured in the model. The model indicates negligible wedge ice abundance in some areas of the Hudson Bay Lowlands where dense polygonal networks are visible in satellite imagery. These areas emerged in the mid-Holocene to late Holocene and include forest tundra vegetation, which both lead to low accumulated wedge ice values in the model. In reality, frequent ice-wedge cracking and rapid wedge growth may be facilitated in these plains where the organic cover is thick and there is ample water supply to effectively feed ice vein growth (Allard, 2018).

Finally, the models do not account for the difference in ground ice accumulation due to the mode of permafrost aggradation, i.e., epigenetic or syngenetic. Syngenetic permafrost, which forms contemporaneously with aggrading sediment or organic cover, is commonly more enriched with ground ice than epigenetic permafrost (French and Shur, 2010). In Canada, this has implications for ground ice formation and preservation, particularly where surface organic matter has accumulated during the Holocene. The thermal properties of peat favour aggradation of the permafrost table and the development of thick layers of segregated ice (Burn, 2004; Zoltai and Tarnocai, 1971). In addition, the presence of a surface organic layer may preserve permafrost and underlying inactive ice wedges in environments that no longer support ice-wedge cracking (Shur and Jorgenson, 2007). National-scale datasets of contemporary peatland distribution and thickness are available (Tarnocai et al., 2011), but they do not depict the timing of peat initiation or accumulation over time. Therefore, we do not explicitly consider the important influence of peat accumulation in our models. However, peat accumulation is strongly associated with the hydraulic conductivity of the mineral substrate. Clays and silts have significantly lower hydraulic conductivity than coarser-grained materials, so peat forms preferentially over fine-grained deposits (Tarnocai et al., 2011). Thus, the higher ground ice values assigned to fine-grained lacustrine, marine, and sedimentary bedrock-derived till units indirectly account for the influence of surface organic matter on ice content and the preservation of permafrost. In addition, thick and widespread organic deposits are included in the surficial geology dataset. Nonetheless, the accumulation of peat may be better incorporated in future modelling.

Aggrading fine-grained sediments may also include layers of ice-rich, syngenetic permafrost (Fortier and Allard, 2004; Kanevskiy et al., 2011). Thick Pleistocene loess deposits, sometimes termed “yedoma”, occur in the Klondike region and southwest Yukon, preserving syngenetic wedges and segregated ice (Fortier et al., 2018; Fraser and Burn, 1997; Kotler and Burn, 2000). The ice in these deposits formed prior to 17 ka and may extend to >20 m in depth and is thus not represented by the modelling. Though the distribution of yedoma in Canada is limited to areas of Yukon, the very ice-rich nature of the deposits makes them important to consider in a landscape change context. These deposits could be identified in future ground ice mapping products. Other deposit types, such as alluvial fans, are not differentiated at the scale of the surficial geology dataset, so syngenetic ground ice accumulation at depth in these settings is not represented in the models.

5.1.3 Input value calibration

Calibration includes the adjustment of model parameters to improve agreement between model outputs and observations. The development of the models was an iterative process that included multistage assessments of outputs and small adjustments as knowledge of the mapped surficial material units, paleovegetation distributions, and current ground ice conditions was gained. The assignment of values for surficial material units benefitted from conversations with experts on the surficial geology of northern Canada (Dan Kerr, personal communication, 2017, 2018) and examination of map legends from larger-scale surficial geology maps. Volumetric estimates of ground ice, themselves derived by upscaling from relatively few quantitative field data, are available from only a few areas in Canada, including Richards Island, Banks Island, and Fosheim Peninsula (Bernard-Grand'Maison and Pollard, 2018; Couture and Pollard, 1998; French, 1974; Pollard and French, 1980). Therefore, there is insufficient field data to meaningfully calibrate model parameters and quantitatively validate the outputs. It is for this reason that we maintain qualitative descriptors for the ground ice abundance rather than numeric estimates in the map legends.