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The transition zone exhibits the effects of cryoturbation, contains abundant
redistributed organic carbon, is enriched in ice in the forms of lenses, veins and
reticulate vein ice (nets), and has abundant soil moisture. In Arctic Alaska, the sur-
face of the transition zone was found at an average depth of 34 ᄆ 7 cm below the
ground surface and had an average thickness of 23 ᄆ 8 cm (Bockheim and Hinkel
2005 ). They observed no significant differences in the depth of the boundaries and
thickness of the transition zone in drained thaw-lake basins ranging in age between
300 and 5,500 year BP, suggesting that the processes leading to its development
occur rapidly in arctic Alaska. Recognition of the transition zone has implications
for understanding pedogenic processes in permafrost-affected soils and for deter-
mining the response of near-surface permafrost to climate warming.
2.4
Permafrost
2.4.1
Definition of Permafrost
Permafrost is defined as a condition—and not a material—in which a material
remains below 0 ᄚC for 2 or more years in succession (Muller 1943 ; van Everdingen
1998 ). Traditionally, this definition applies to rocks and buried ice as well as uncon-
solidated sediments, but it does not apply to glaciers, icings, or bodies of frozen
surface water. Permafrost is synonymous with cryotic ground.
There are two general kinds of permafrost, including ice-cemented and dry-
frozen. In the latter case, the material is below freezing but contains insufficient
pore ice to be cemented. Ground ice is a general term referring to all types of ice
contained in freezing and frozen ground.
2.4.2
Permafrost Boundaries and Distribution
Three zones of permafrost have been delineated, including continuous where per-
mafrost covers 90 % or more of the surface, discontinuous where it covers between
50 and 90 %, sporadic where it covers 10-50 %, and isolated where it covers less
than 10 % of the surface (Fig. 2.5 ). Permafrost covers 22.0 × 10 6 km 2 , 24 % of the
Earth's exposed land surface, but it likely covered up to 40 % during major glacia-
tions (French 2007 ). About 99 % of the world's permafrost exists in the Northern
Hemisphere (Fig. 2.5 ). Countries with the greatest areas of permafrost include the
Russian Federation (11.0 × 10 6 km 2 ), Canada (6.0 × 10 6 ), China (2.1 × 10 6 ), and the
USA (1.1 × 10 6 km 2 ) (Gruber 2012 ). In the Southern Hemisphere, permafrost occurs
in the southern Andes (100,000 km 2 ) and ice-free areas of Antarctica (49,500 km 2 )
(Fig. 2.6 ). The distribution of permafrost is controlled by climate, elevation, latitude,
snow cover, and other parameters.
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