Frozen ground

Perennially frozen ground, known as permafrost, underlies nearly all of the Arctic land area. Permafrost is said to be present whenever ground temperatures are below freezing through two summer seasons. Ground ice need not be present, although in sediments ice may be present either in segregated form throughout, or as lenses and wedges (French, 1996). The upper part of the ground in permafrost regions, termed the active layer, thaws seasonally. The active layer depth (the maximum thaw depth) depends on air temperature, vegetation cover, soil material, moisture content and the degree of insulation provided by the winter snow cover. The active layer depth typically varies from less than 50 cm in the coldest regions of the Arctic to 150 cm or more in warmer southern regions. Active layer depths are greater on south-facing slopes and in coarse-textured soils, and least in peat bogs (Chernov and Matveyeva, 1997).

In acting as an impermeable barrier, permafrost plays a major role in the hydrology of the Arctic. Following snowmelt, the ground becomes waterlogged unless there is sufficient topographic relief for drainage channels to form. Hence there are numerous shallow lakes. Permafrost is generally absent beneath large deep lakes, especially in the sub-Arctic. As will be outlined in Chapter 6, in areas with sufficient drainage, the impermeable permafrost barrier fosters rapid channeling of precipitation and meltwater into streams, which impacts strongly on the seasonality of river discharge and evapotranspiration rates.

Zones where permafrost may be present account for 24% of the Northern Hemisphere land surface (Zhang et al., 1999). However, the actual area underlain by permafrost is smaller if we take account of the fractional coverage of different permafrost zones (continuous, 90 to 100%; discontinuous, 50 to 90%; sporadic, 10 to 50%; isolated, less than 10% of the ground underlain by permafrost). On this basis 12.8-17.8%

Figure 2.13 Northern Hemisphere permafrost distribution, simplified from the International Permafrost Association map. Black areas indicate continuous permafrost, striped areas discontinuous permafrost, and stippled areas sporadic permafrost (courtesy of C. Oelke, NSIDC, Boulder, CO).

of the land surface is underlain by permafrost (Zhang etal., 2001). The area mapped as continuous permafrost represents 47% of the entire permafrost region of the Northern Hemisphere. In addition, there is frozen ground below much of the Greenland Ice Sheet and the subpolar ice caps.

The Northern Hemisphere permafrost distribution, shown in simplified form in Figure 2.13, was compiled by the International Permafrost Association (IPA) (Brown et al., 1997). Permafrost is spatially continuous where the mean annual SAT is below approximately —7 °C, and is discontinuous for an SAT of —1 °C to —3 °C. The permafrost thins towards the southern borders, near the —1 °C SAT isotherm (Ives, 1974). In the zone of continuous permafrost, the thickness exceeds 600 m in northeastern Siberia and in northern Canada and Alaska. Extreme depths of 1200-1470 m are found in the Anabar Plateau of north-central Siberia (72-73° N, 113° E) (Tumel, 2002). This is attributed to ice-free conditions through the Quaternary, low geothermal heat fluxes and high bedrock conductivity. Southward from the zone of continuous permafrost in Siberia and Canada, and also in Scandinavia, there is extensive discontinuous permafrost. The typical thickness is of the order of 50 m in the discontinuous zone, but only a few meters where the distribution is sporadic. Exposed treeless ridges and north-facing slopes are underlain by permafrost whereas it may be absent in forested valleys, especially where there are large lakes. In general, the permafrost thins southward and becomes sporadic. Eventually it is found only in islands in favorable locations. Some of these permafrost islands may be relics that are unstable under the present climate.

North of the Arctic coastline there is a considerable zone of submarine permafrost underlying shallow sections of the continental shelf seas (Vigdorchik, 1980). This zone is extensive in the Barents, Kara, Laptev and East Siberian seas, but is narrower in the Chukchi and Beaufort seas. Gavrilov (2001) proposes that most submarine permafrost represents relic strata formed during the glacial intervals of the late Pleistocene, when lower sea levels exposed the shelf areas and mean winter temperatures were well below modern values. As sea levels rose during the subsequent Holocene, the upper ice-rich strata were thermally abraded.

The modern occurrence and characteristics of submarine permafrost are determined by a number of factors. These include the water mass structure and vertical circulation, the absorption of solar radiation in summer when the coastal seas are ice-free, and the corresponding heat loss to the atmosphere in winter through polynyas and leads, geothermal heat flow, the bottom water temperature, salinity and the freezing of bottom sediments by grounded landfast ice and stamukhi (grounded ridges). Seaward of the submarine permafrost, there may also be a narrow zone of seasonally frozen submarine sediments. In the Beaufort Sea, the permafrost upper boundary varies between the seabed and 100-150 m and its base may lie as deep as 600-900 m. In the Laptev Sea region (Hubberten and Romanovskii, 2001) permafrost is continuous to the -65 m isobath, and discontinuous to the -100 to -120 m depth. The maximum submarine permafrost thickness is about 500 m north of Kotelny Island.

The ice content of permafrost is difficult to determine quantitatively, due to its spatial variability and the limited number of boreholes allowing for analysis. Only about 10% of the terrestrial permafrost area is "ice rich". The IPA map shows that ice-rich permafrost, with an ice content >20% by volume in the upper 10-20 m of the ground, is widespread in the coastal Siberian lowlands of Yamal (around the Ob estuary) and east of the Lena Delta in Yakutia.

Surface icings (aufeis, naled, taryn) represent a special component of ground ice. They form when surface water from springs and groundwater seepage freezes, due to persistent low temperatures, or when rivers overflow their banks, sometimes as a result of ice jams. They are most extensive in Siberia, but they also occur in Alaska, mainly east of the Colville River on the Arctic coastal plain (Harden et al., 1977; Hall and Roswell, 1981). The total area of aufeis for all of northern Russia is estimated to be 128 000 km2 (Kotlyakov, 1997). The largest single feature in northeastern Siberia (120 km2) extends for up to 26 km along the valley of the Momy River (Golubchikov, 1996).

2.2.4 Lakes

Lakes are an important element of the Arctic and sub-Arctic landscape. Bonan (1995) estimates that water surfaces (including wet muskeg bogs and swamps) occupy up to

20-40% of the landscape in the glacial drift-covered lowlands of northern Canada and western Siberia (Figure 2.14). In northern Canada there are several lakes larger than 1000 km2 in area. Great Bear Lake (31 000 km2) and Great Slave Lake (28 500 km2) are by far the largest and both exceed 400 m in depth. By contrast, Lake Netilling on Baffin Island is just over 5000 km2 and Lake Hazen on Ellesmere Island (81.5° N, 70° W) is 540 km2. In Finnish Lapland, the largest is Lake Inari (69° N, 27.5° E) at over 1000 km2. There are few major lakes in the Russian Arctic. Lake Taymyr (74° N, 74° E) has an area of about 4500 km2. In the tundra areas of western Siberia and the Taymyr Peninsula, lakes are typically smaller than 0.3 km2, with a few over 50 km2. Here they represent 2-5% of the surface (Ananjeva, 2000), but in the coastal plain of northern Alaska so-called thaw lakes form about 40% of the surface. In Greenland, the Arctic islands, and the mountains of Alaska and the Yukon (Post and Mayo, 1971), there are many ice-dammed lakes. Many of them periodically drain and subsequently refill. Hence they are ephemeral features of the landscape.

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