Friday, 14 July 2006
106-8

Soil Formation in Frost-boil Environments.

Yuri Shur1, Chien-Lu Ping1, and M. Torre Jorgenson2. (1) University of Alaska Fairbanks, 237 Duckering, Fairbanks, AK 99775-5900, (2) Alaska Biological Research Inc, PO Box 80410, Fairbanks, AK 99708

Frost boils are features that have a round bare surface, are slightly elevated above the surrounding ground, are bowl shaped at the bottom, and have a continuous or broken layer of Organic Matter (OM) at the periphery of the active layer. Frost boils and the vegetation that develops in the surrounding inter-boil areas form a well-organized and long-term functioning system that includes the upper permafrost. Formation of this system starts with the development of shallow thermal contraction cracks within the active layer, followed by colonization of vegetation in the moister, snow protected microsites of the cracks. These small polygons that develop mainly in the active layer are common features of barren ground and have been described on numerous occasions. Subsequent vegetation growth, decomposition, and OM accumulation in the limited areas next to cracks change the thermal properties of the active layer below the vegetated parts of surface. Consequently, the active layer steadily decreases, leading to formation of aggradational ice and perennial frost heave beneath vegetated inter-boils. Due to the three-dimensional nature of the freezing front in winter, 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. Ross Mackay showed that the bowl shape controls the movement of the thawed material from the periphery to the beneath the central part of the boil through gravitational processes. Of particular interest, is the accumulation of OM along the permafrost table surrounding the bottom of the frost boil. Previous studies have variously suggested that this accumulation is due to: (1) residual accumulation of dissolve organics moving through the mineral soil to the permafrost table during the previous stage when surface of the contemporary boils was vegetated, (2) convective circulation within the boil, or (3) sedimentation and burial of organics. Instead, we propose four-stage process. First, ice-rich cryogenic structures develop along the bottom of the frost boil during annual freeze back and as a longer-term transient layer at the top of the cold permafrost table. Second, pressure from the expansion of the freezing boil squeezes the surrounding organic matter, which is confined by the frozen surface layer, and displaces it downward. Third, the thin ice-rich soil at the bottom of the boil occasionally thaws in the spring leaving a void around the boil. Finally, the moderately to highly decomposed OM, which forms and accumulates in inter-boils areas, creeps or flows with water down into the void. In a closely related process, dissolved OM associated with this subsurface accumulation moves toward the boil surface with water that migrates to the surface freezing front in winter and to the drying surface in summer. This process results in formation of a three-dimension soil profile with O horizons surrounding the boil and an A horizon within the lower mineral layers of the boil. Inside the frost boil, this accumulated humic material moves in the direction from the bottom toward the center and the upper part of the boil. With time, the organic layer often completely covers the periphery of boils. Vegetation succession and accumulation of OM at the surface decrease active-layer thickness. This, in turn, causes increased accumulation of aggradational ice, incorporation of the previously intruded OM underneath the frost boil into the upper layer of permafrost, and increased perennial heave of the surface. 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. Removal of vegetation from the earth hummocks leads to thawing of aggradational ice and thaw settlement.

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