Ninth International Conference on Permafrost ... - IARC Research

Formation of Frost Boils and Earth HummocksYuri ShurUniversity of Alaska Fairbanks Department of Civil and Environmental Engineering, Fairbanks, Alaska, USATorre JorgensonABR – Environmental Research and Services, Inc., Fairbanks, Alaska, USAMikhail KanevskiyUniversity of Alaska Fairbanks Institute of Northern Engineering, Fairbanks, Alaska, USAChien-Lu PingUniversity of Alaska Fairbanks Agriculture and Forestry Experiment Station, Palmer, Alaska, USAExisting hypotheses of frost boils and earth hummocksformation generally are limited to climatic and active layerprocesses within a simple two-component system. Based onour field studies and literature review, we have identifiedtwo other important factors: vegetation, which affects activelayer depth and organic-matter accumulation, and permafrostaggradation, which affects heave and thaw settlement.Accordingly, we have developed a conceptual model of afour-component system involving climate, vegetation, activelayer, and permafrost.A model consists of five stages in the development of frostboils and earth hummocks (Fig. 1). The first three stagesdescribe formation of frost boils, which generally occur inhigh- to mid-arctic tundra ecosystems. The last two stagesdepict the evolution of frost boils into earth hummocks,which typically occurs under the slightly warmer climates ofthe low arctic and taiga regions, or in response to warmingclimatic conditions.Figure 1. Stages of frost boils and earth hummocks formation.Seasonal segregated ice within the active layer is not shown.First, small (0.5–3 m) polygons form under a bare soilsurface due to frost cracking. This process is typicallylimited to the high- to mid-arctic, where the low temperaturesand thin snow allow sufficient contraction cracking. Thecontraction cracking generally is limited by the thickness ofthe active layer because of the small-scale of the features, asopposed to the deeper cracking associated with the largerscalecontraction associated with development of ice-wedgepolygons. These small polygonal forms are widespread inthe Arctic as noted by ecologists, pedologists and permafrostscientists.Second, vegetation colonizes the protected microsites ofthe shallow troughs that develop over the cracks. The surfaceof frost-boils is usually elevated above the surroundinginter-boil areas, and therefore, is susceptible to wind erosion,especially during winter freeze when the surface becomeselevated by seasonal frost heave. Needle ice preventsvegetation colonization of polygon centers.Third, further vegetation growth and organic-matteraccumulation in troughs change the thermal properties ofthe soil, causing the depth of the active layer to steadilydecrease. In response, segregated ice forms at the top of theaggrading permafrost table, creating an intermediate layerwith distinctly different soil and ice morphology (Shur1988). Gravimetric moisture content in this layer oftenexceeds 100%. The aggrading ice causes the ground surfaceto heave. This perennial frost heave differs from seasonalfrost heave in the active layer, because it is cumulative andleads to formation of long-term existing features. Featuresformed by perennial frost heave exist for tens to thousands ofyears. The differential relief caused by perennial frost heavein frost-boil systems varies from centimeters to decimeters.At this stage, movement of organic matter from troughsunder frost boils becomes important. The permafrost tabledevelops ridges beneath the vegetated areas and concavedepressions, or “bowls,” beneath the frost boils. Because thepermafrost table slopes at the margins of the bowls, organicmatter formed in inter-boil areas creeps or flows undersaturated conditions into the voids and cracks. There aretwo possible mechanisms for this gravitational movement ofsaturated organic matter down to the permafrost table. First,contraction cracks continue to develop, particularly at theinterface of the organic matter and the mineral soil. Duringsummer thaw, saturated organic matter, which tends to bemoderately or highly decomposed, flows down the cracks.287