Widespread Degradation of Ice Wedges on the Arctic Coastal Plain in Northern Alaska in Response to the Recent Warmer Climate

The continuous permafrost on the Arctic Coastal Plain in northern Alaska has been considered stable because permafrost temperatures remain low, even with an increase of several degrees during the last decades. Ice wedges, however, are particularly susceptible to degradation because only a very thin layer of permafrost (the transient layer) exists between the ice and the bottom of the active layer. An increase in the active layer during unusually warm periods causes the thawing front to encounter the underlying ice wedges and initiate degradation. Field observations and photogrammetric analysis of 1945, 1979, and 2001 aerial photography indicate that there has been widespread degradation of the ice wedges on the Arctic Coastal Plain west of the Colville Delta over the recent 57-year period, and indications are that most of the degradation occurred during the last two decades. Field sampling at 46 polygonal troughs and their intersections showed that ice wedge degradation has been relatively recent as indicated by newly drowned vegetation. We found thermokarst was widespread on a variety of terrain conditions, but most prevalent on, ice-rich centers of old drained lake basins and alluvial-marine terraces, which have the greatest ice wedge development in the studied landscape. Ice wedges on these terrains typically occupy from 10 to 20 % of the upper permafrost. We attributed the natural degradation to warm weather during the last decades, because disturbance of the ground surface, which could have similar impact on ice wedges, was not evident. While, ice-wedge degradation probably has been periodically occurring at low rates over the preceding centuries, it has greatly accelerated during the last several decades. We identified six stages of ice-wedge degradation and stabilization. They include: (1) the loss of transient layer of upper permafrost above ice wedges, leading to enhanced nutrient availability and vegetative growth; (2) thawing of ice wedges and surface settlement; (3) accumulation of water in degrading troughs and killing of vegetation; (4) increased thawing of ice wedges under flooded conditions, which is often accompanied by lateral thermal erosion of ice-rich soil along polygon margins; (5) initial establishment of aquatic mosses and sedges; and (6) stabilization of the ice wedges through robust vegetative growth, accumulation of organic matter, and reestablishment of permanently frozen soil. The maximum depth of thaw settlement at ice wedge intersections, where degradation is greatest, reaches about 1.5 m. During re-stabilization, clear congelation ice often forms above ice wedges and pseudomorphs may form as a cast in deeper thawed portions of the ice wedges. The degradation of ice wedges greatly affects ecosystem properties by changing the micro-relief, causing inundation of the settling surface, altering vegetation, and enhancing methane production under anaerobic conditions. It remains uncertain, however, whether ice-wedge degradation can lead to development of larger thermokarst lakes. Development of waterbodies larger that the thermokarst pits at wedge intersections was not evident and evidence indicates that the degraded ice wedges stabilize before the central parts of polygons are substantially degraded.