High-centered polygons in the So/r Rondane Mountains, East Antarctica: Possible effect of ice wedge sublimation

概要

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Small high-centered polygons, 3-15m in diameter, dominate moraine fields in an inland cold desert of the Sr Rondane Mountains, Antarctica. They mainly occur on moraine fields at two stages younger than 1Ma. The polygons on the younger moraine (<0.5Ma) generally have an ice wedge surrounded by ice-cemented permafrost, although the ice wedge cracking is inactive or too slow to be detectable. The polygons on the older moraine (0.5-1Ma) have either an ice wedge or ice-free wedge. The ice-free wedge underlies a subsided trough and consists entirely of loose and coarse sediments with vertically-oriented clasts, which represents an ice-wedge cast probably originating from long-term sublimation of an ice wedge. These observations suggest that flat-top polygons with ice wedges form in wet permafrost when located close to the ice sheet surface, but that the subsequent ice sheet lowering separates the polygons from the moisture source, and finally long-term ice sublimation leads to domed polygons with ice-wedge casts enclosed in dry permafrost.