Ninth International Conference on Permafrost ... - IARC Research

Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tSecond, the thawing of segregated ice, which is formedduring fall freeze-back at the top of the permafrost table,leaves voids that are filled with mobile organic matter. Overtime, a concave layer of organic matter mixed with mineralsoil accumulates at the permafrost table underneath themineral frost boil. In some boils the organic matter can bediscontinuous, but can be traced to its source—the vegetatedinter-boil area.Penetration of organic matter along the bottom of theactive layer leads to further decrease in the active layer,aggradation of ice, and incorporation of organic matter inthe upper permafrost. This in turn further raises the surfaceof the frost boil and increases differentiation in seasonalfrost heave between boils and inter-boil areas. At this stage,a mature frost boil has developed, and can become theclimax stage in the Arctic. While this downward movementof organic matter around frost boils is similar to elements ofcirculation as described by Mackay’s model (1980), thereare important differences. In our model, organic moveddownward accumulates in the aggrading permafrost. Thereis no upward movement of solid material, and only dissolvedorganic can move to the freezing front in winter or desiccatedsurface in summer.Fourth, frost boils evolve into earth hummocks whenvegetation and organic-rich soil eventually expand to coverthe entire surface. This spread may occur over a long timeand may be assisted by climate warming. The spread leads tofurther decreases in depth of the active layer and additionalaccumulation of aggradational ice above the layer of organicmatter, with more accumulation of aggradational ice andmore perennial frost heave.Fifth, sufficient vegetation develops, peat accumulates,and seasonal thaw no longer reaches the base of the formerfrost boil. Eventually, the thawing front can become limitedto the surface peat and no longer penetrates the mineral frostboil. Our previous studies (Shur & Ping 2003, Shur et al.2005, 2006) explained evolution of the earth hummocksfrom the frost boils due to growth of vegetation at the surfaceof frost boil and subsequent accumulation of aggradationalice. Recent observations by Kokelj et al. (2007) support suchan explanation.Earth hummocks once fully developed are very sensitive toenvironmental changes because they evolved in conjunctionwith ice aggradation below the active layer. Disturbance tothe surface, such as by fire, or climate warming can leadto degradation of the extremely ice-rich soil beneath theactive layer. In a time scale of hundreds to thousands ofyears, climate change is the most important process leadingto the degradation of the permafrost and earth hummocks(Fig. 2A). In a time scale of years, denudation of vegetationis the leading process in the formation of regressive frostboils. Vegetation recovery, however, can reverse the processand stabilize or restore the earth hummocks (Fig. 2B). Suchprocesses were evident in field observations by Kokelj et al.(2007).AcknowledgmentsWork was supported by NSF grants ARC-0454939 (toYS), ARC-0454985 (to MTJ), EPS-0346770 (to MZK), andOPP-0120736 (to CLP).ReferencesKokelj, S.V., Burn, C.R. & Tarnocai, C. 2007. The structureand dynamics of earth hummocks in the subarcticforest near Inuvik, Northwest Territories, Canada.Arctic, Antarctic and Alpine Research 39: 99-109.Mackay, J.R. 1980. The origin of hummocks, western Arcticcoast, Canada. Canadian Journal of Earth Science17: 996-1006.Shur, Y.L. 1988. The upper horizon of permafrost soils.Proceedings of the Fifth <strong>International</strong> <strong>Conference</strong> onPermafrost, Trondheim, Norway, August 2–5, 1988:867-871.Shur, Y., Ping, C.L. & Jorgenson, M.T. 2006. Soil formationin frost-boil environment. Proceedings of 18th WorldCongress of Soil Science, Philadelphia, Pennsylvania,USA. July 9–15, 2006. Abstract 106-8.Shur, Y.L., Ping, C.L. & Walker, D.A. 2005. Comprehensivemodel of frost boils and earth hummocks formation.Proceedings of the Second European <strong>Conference</strong> onPermafrost, Potsdam, Germany, June 12–16, 2005:79.Shur, Y. & Ping, C. 2003. The driving force of frost boils andhummocks formation. Eos Trans. AGU 84(46), FallMeet. Suppl., Abstract C21B-0823.Figure 2. Degradation of earth hummocks due to (A) permafrostdegradation and (B) denudation of vegetation, resulting in theregressive frost boils formation. 1 – vegetation; 2 – peat, organicmatter; 3 – permafrost table; 4 – ice lenses.288