CYCLIC DEVELOPMENT AND THERMOKARSTIC DEGRADATION OF PALSAS IN THE MID-ALPINE ZONE AT LEIRPULLAN, DOVREFJELL, SOUTHERN NORWAY

The palsas of Leirpullan are developed in deltaic sands and silts in t he zone of alpine permafrost at an altitude of 1437 m (estimated mean annual air temperature -2.7 degrees C). Five mounds that represent a c hronosequence are analysed with particular reference to initiation, ag gradation and degradation phases. It is inferred that geoecological fa ctors control initial mound formation beneath areas of thin (<20 cm) m oss-peat, which provide summer insulation for developing permafrost. L ow summer temperatures and low summer precipitation appear to allow th e survival of permafrost, the existence of stable mounds, and limited height growth, after the gradual successional replacement of moss-peat by fellfield vegetation growing directly on the mineral substrate. De gradation occurs owing to thermokarstic processes, which result in the formation of rim-ridge ramparts. 'Exogenous' processes associated wit h lateral thermal erosion by lake and river water are important at the site. Regeneration and the possible effects of climatic change are al so discussed. Palsas with only a thin peat cover in the initial stage of formation but no peat cover during subsequent development and degra dation are considered to be near one extreme of a morphological and ge netic continuum of pure peat palsas to mineral permafrost mounds. The rim-ridge ramparts characteristic of the degraded palsas of Leirpullan appear to provide appropriate modern analogues for relict Pleistocene forms in Wales and elsewhere in Europe formerly interpreted as pingo scars. (C) 1997 by John Wiley & Sons, Ltd.