HR: 0830h
AN: C21B-0823    [PDF]
TI: The Driving Force of Frost Boils and Hummocks Formation
AU: Shur, Y
EM: ffys@uaf.edu
AF: Department of Civil and Environmental Engineering, University of Alaska Fairbanks, PO Box 755900, Fairbanks, AK 99775 United States
AU: * Ping, C
EM: pfclp@uaa.alaska.edu
AF: School of Natural Resources and Agricultural Sciences, University of Alaska Fairbanks, 533 E Fireweed, Palmer, AK 99645 United States
AB: Formation of frost boils has several aspects to be explained, including the bowl shape of boils, the formation of an organic layer at the periphery of the frost boils, the elevated center of the boils, and resistance of the soil surface to vegetation colonization. Genesis of frost boils and hummocks have been widely attributed to cryoturbation-a complex of seasonally interchanging processes of frost heave and thaw settlement. Existing hypotheses, however, do not consider changes in the upper permafrost as factors of frost boil and hummock formation. Most of features listed above cannot be explained by reversible seasonal changes in the active layer. Frost boils and inter-boil areas form a well-organized and long-term functioning system closely linked to upper permafrost dynamics. Formation of this system starts with the occurrence of vegetation in inter-boil areas in shallow thermal cracks limited by the thickness of the active layer. Vegetation and its decomposition change the thermal properties of the active layer that steadily decreases with formation of aggradational ice and perennial frost heave beneath vegetated inter-boils. Due to the three-dimensional nature of the freezing front, frost heave in inter-boil areas is not perpendicular to the soil surface but is inclined in the direction of the boils, thus forming the bowl shape of boils. The organic matter formed in inter-boil areas intrudes under boils due to formation of frost cracks and thawing of ice lenses and layers at the bottom of the active layer. Penetration of organics along the active layer - permafrost boundary from an inter-boil area leads to formation of additional aggradational ice in the intermediate layer at the top of the permafrost accompanying the intruding organics. This aggradational process reinforces the bowl shape of frost-boils. Accumulation of organic matter at the lower part of the active layer leads to a decrease in active-layer thickness under the boils, which in turn leads to accumulation of aggradational ice and perennial irreversible frost heave. This increases the difference in elevation of the surface between frost boils and inter-boil areas. Vegetation colonization is then impeded by the elevation differences and formation of needle ice on the surface of the exposed mineral soil. With time, the organic layer often encircles completely the periphery of boils. The net result is carbon sequestration at the active layer-permafrost boundary. The organic matter at the bottom of the active layer is not accessible to cryoturbation. To be susceptible to seasonal frost heave, the organic matter accumulated at the bottom of the active layer has to be pushed upward in to cracks in the mineral soil, which occasionally form at the bottom of the active layer with the opening toward the bottom. Active cryoturbation of mineral and organic soil throughout the entire active layer by seasonal frost heave and thaw settlement, as is postulated in most existing explanations of frost-boil formation, have never been evident in our field observations. In the continuing evolution of frost boils, vegetation development on the boil surface and readjustment of the upper permafrost lead to the development of hummocks. The development of vegetation and accumulation of organics at the surface leads to decreasing active-layer thickness. This in turn causes increased accumulation of aggradational ice, incorporation of the previously intruded organics underneath the frost boil into the upper layer of permafrost, and increased perennial heave of the surface.
DE: 0400 Biogeosciences
DE: 1823 Frozen ground
DE: 1824 Geomorphology (1625)
SC: Cryosphere [C]
MN: 2003 Fall Meeting