The concept of gelic materials in the new Gelisol order for permafrost-affected soils

By taxonomic definition, soils in the recently adopted Gelisol order in Soil Taxonomy must contain permafrost within 100 cm of the soil surface or gelic materials within 100 cm of the surface and permafrost within 200 cm of the surface. Five pedons representative of Gelisols are used to illustrate the nature and properties of gelic materials. Gelic materials may be either mineral or organic and are affected by cryoturbation (frost churning), ice segregation, and/or thermal contraction cracking. Gelic materials may occur in any horizons of the active (seasonally frozen) layer and/or the upper permafrost. Cryoturbation is manifested by irregular and broken horizons, involutions, organic matter accumulation on top of and within the permafrost, oriented coarse fragments, and silt-enriched layers. Ice segregation is manifested by ice crystals, ice lenses, vein ice, and ice wedges. Thermal cracks may be filled with ice or soil material. In surface mineral horizons, gelic materials contain granular, platy, and vesicular structures; subsurface horizons have blocky and prismatic structures or structureless, massive conditions. There are distinct soil fabrics in gelic materials that are visible in thin sections, e.g., orbiculic, suscitic, conglomeric, banded, and granulitic, that reflect the cryopedogenic processes of cryodesiccation, displacement, and pore formation. Cryopedogenic processes that lead to gelic materials are driven by the physical volume change of water to ice, moisture migration along a thermal gradient in the frozen system, or thermal contraction of the frozen material by continued rapid cooling.