The structure and morphology of minerogenic palsas in northern Norway

Abstract

area in northern Norway the outer coastal region has an annual mean temperature of 1-3°C, while the inner fjord region has a temperature around 0°C. The most continental area is in the southeast Where the mean annual temperature varies between -2° and -3°C. Where the mean annual temperature is below 0°C palsas are numerous in bogs. In the more continental region, permafrost has been found even outside bogs, though little is known about its areal extent. From airphoto analyses and field work a morphological classification of palsas in different main types has been possible. Palsa plateau: low palsas 1.0--1.5 m high, covering areas from 1000 m² to 1 km². A thin peat layer and a mineral core with low clay and silt content. Esker palsa: ridges 2-6 m high and 50-500m long, parallel to the bog inclination. A mineral core of a clayey silt and' fine sand. String palsa: ridges 1-2 m high and 25-100m long, perpendicular to the bog inclination, develop often from fen strings and are normally built of pure peat but can have a mineral core if the peat layer in the bog is thin. Conical palsa: 2-6m high with a circular to oval basal circumference. Mostly pure peat palsas with no minerogenic core. Palsa complex: large palsa areas of different types and stages of development. Heavy erosion and disintegration occur close to growing and newforming palsas. The height varies from 1-9 m over the bog surface. Best developed when a thin peat layer covers a mineral core with a high clay and silt content. The grain size distribution in the minerogenic core is usually a clayey silt with a clay content between 10-30% and silt between 40-60%. In palsa plateaus, however, the clay content is often no more than 2-6% and fine to medium sand constitutes 60-80%. The ice (water) content largely exceeds the capillary capacity of the soil. In a section through a palsa complex the following values of the water content were measured: 36, 67, 87, 114, and 178% of the dry weight of the soil.

Palsas are generally considered to have a peat cover which insulates the frozen corele during the summer. The physical processes, however, are the same whether the palsa formation takes place in a peat bog or in a pure mineral soil. The missing peat layer must then be compensated for by a lower summer temperature.