Although peatlands are a major landscape component in subarctic Fennoscandia and their contribution to the global carbon budged is well acknowledged, detailed studies on their initiation and lateral and horizontal expansion in this area are still scarce. Here we present new data on the initiation and lateral expansion of five small sub-arctic peatlands in the Fennoscandian tree-line region, based on 21 AMS-radiocarbon dated basal peat samples.

The cumulative number of basal peat AMS radiocarbon dates can be divided into four distinctive periods (Fig 1): 1) the initiation and rapid expansion of mires starting after deglaciation between ca. 10000 to 8000 cal yr BP, 2) slow expansion and modest formation of new peatlands between ca. 8000 and 4000 cal yr BP, 3) an extensive lateral spread and forming of new peatlands between ca. 4000 to 3000 cal yr BP, and 4) slow expansion and modest formation of new peatlands from ca. 3000 cal yr BP to present. These periods are in agreement with the larger scale Holocene climate patterns in the area, namely the relatively warm and moist early Holocene, the Holocene thermal maximum (ca. 8000 to 4000 cal yr BP) and the neoglacial cooling from ca. 4000 cal yr BP to present.

The initiation and rapid expansion of peatlands in the high latitude regions after deglaciation (ca. 11000 to 8000 cal yr BP) has also been documented by e.g. MacDonald et al. (2006) and Mäkilä & Moisanen (2007). However, according to their results the bulk of peatlands has already formed before ca. 5000 cal yr BP, after which no major initiations and expansions of peatlands have occurred. Our study suggests that a second period of initiation and/or expansion of peatlands in northern Fennoscandia occurred between ca. 4000 to 3000 cal yr BP. This result is also supported by Korhola (1995) who studied 23 mires in southern Finland and concluded that the most intensive period of peatland formation in this area occurred between ca. 4300 and 3000 cal yr BP.