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Rhyolitic and dacitic component of the Askja 1875 tephra in southern and central Finland: first step towards a Finnish tephrochronology

Tephra from Icelandic volcanic eruptions is frequently dispersed to northern Europe, but so far tephra has not been detected in Finnish sedimentary records. We report the first finding of a cryptotephra layer in southern and central Finland. Sediment samples from five lakes and 10 peatlands, most of them located in the tephra fallout zone of the Hekla 1947 eruption, were investigated for the presence of cryptotephra to assess the potential for tephrochronology in Finland. Tephra shards were extracted from their host matrix and electron probe microanalysis was conducted on single shards for geochemical characterization. Our results confirm the presence of Askja 1875 tephra in Finland, thus extending the known dispersal area of the Askja 1875 tephra eastwards. Most of the shards are rhyolitic, but a minor concentration of tephra with dacitic composition forms a narrow dispersal zone extending from Sweden to southern Finland. This zone possibly represents the main dispersal axis of the tephra in the distal area. Our results suggest that Finnish environmental research could benefit greatly from adding tephrochronology to the array of dating methods commonly used. Additionally, the absence of Hekla 1947 tephra in the previously inferred fall‐out zone is an important result indicating the complexity of tephra deposition and preservation.