| Summary: | Tephrochronology is a high-precision dating method that uses volcanic ash horizons as isochrons in correlating and dating geological records and archaeological sites. First developed in the volcanic regions of the world, tephrochronology has expanded to ever more distal areas with improved laboratory and analytical methods that have enabled the utilization of even the scarcest deposits of far-travelled cryptotephra i.e. small volcanic glass shards that are invisible to the naked eye. The objective of this dissertation is to assess the potential of cryptotephra studies and tephrochronology in Finland. No cryptotephra studies had been conducted in Finland previously, and the ultimate aim of the work presented here was to establish a first outline for a Finnish tephrochronology that could be used as a dating tool in environmental research in the region. Cryptotephra was searched from 30 peatland and lake sites from an area that covers the whole southern and central Finland from Åland archipelago in the west to the Russian border in the east. As a result, cryptotephra deposits from at least 17 Icelandic and two Alaskan volcanic eruptions were detected and geochemically characterized from the Finnish environmental archives. The oldest identified tephra in Finland is the 7 ka Hekla 5 tephra and the youngest one is the Askja 1875 tephra. The Finnish tephrochronology therefore covers approximately 7000 years and the results of this study demonstrate that dispersal of tephra to Finland has been relatively frequent throughout this time. Within this project, the known dispersal areas of several Holocene tephras, such as Askja 1875, Hekla 1845, Hekla 1510, Landnám (Torfajökull), White River Ash eastern lobe, Hekla Ö and Aniakchak tephra were extended significantly eastwards, and the Hekla Y tephra was identified for the first time outside of Iceland. These results indicate that Icelandic tephra can travel to Finland along complex northerly and southerly pathways in addition to a direct eastwards dispersal route. ...
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