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Stratigraphy, age and formation of peaty earth hummocks (pounus), Finnish Lapland

FRE2255 CNRS, Sedimentologie et Ge'odynamique, Universite Sciences et Techniques de Lille,-SN5, 59655 Villeneuve d'Ascq Cedex, France

Matti Seppala

Physical Geography Laboratory, Department of Geography, Helsinki University, PO Box 64, FIN-00014 Helsinki, Finland; mseppala{at}touko.helsinki.fi

Detailed investigations of peaty earth hummocks called pounus were carried out in northemmost Finnish Lapland in the vicinity of the Kevo Subarctic Research Station. Pounus are complex, often vegetated, cryogenic mounds formed partly of peat. Some have a core of mineral soil or several large stones, or just one frost-heaved boulder covered by peat. They belong to the same group of earth hummocks as thufur and non-sorted circles. The typology of pounus is described and compared with other types of cryogenic earth hummocks. Stratigraphical study of pounus shows how cryoturbation has deformed the previous soil horizons of typical Arctic podzols and has been followed by intense peat growth and later frost reactivation. Microstructures in pounus confirm the frost activity and several phases of peat growth during their formation. Alternating warmer and dryer conditions with water-table rise and renewed peat formation could be inferred. The present stage of development is frost reactivation and peat growth. Pounus are relatively young features. The maximum age for pounu development corresponds with the age of peat formation. Peat of a boulder pounu yielded a data of 330 + 70 BP and a minerogenic pounu gave an age of 710 + 50 BP. The Sphagnum peat formation on peat pounus started intensively about 1000 BP. They are random surficial forms with variable control by frost heave and vegetation growth, independent of the presence or absence of permafrost. They are related to a dynamic snowcover in areas of open birch stands or tundra and their development is polygenetic with alternating frost disturbance and phases of humification or podzolization. Pounus act thermally as a biological refuge in the autumn and spring. At present, perennial frozen cores seem to be common, especially in large peat pounus. Pounu development, as with other earth hummocks, occurs after the main phase of interglacial pedogenesis and can be considered as an indicator of a general cooling trend, developing mostly during the second half of the Holocene. Pounus' relation to climatic change is discussed. Pounus can be considered as an indication of a general cooling trend associated with progressive depletion of summer insolation. Hummock extent is coherent and parallel with trends in permafrost growth, but not directly linked to it.

Key Words: Pounus • peaty earth hummocks • Holocene climatic change • environmental indicator • cryoturbation • periglaciation • micromorphology • Finland • late Holocene

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