Háskóli Íslands

Ph.D. student: Maarit Kalliokoski

Dissertation title: An outline for Finnish Holocene tephrochronology – volcanic ash as a dating method in Finland

Opponent: Nicholas Balascio, Associate professor at the College of William & Mary, Williamsburg, VA, United States

Advisors: Dr. Esther Ruth Guðmundsdóttir, Associate Professor at the Faculty of Earth Sciences, University of Iceland, Iceland
Dr. Timo Saarinen, Professor at the University of Turku, Finland

Also in the doctoral committee: Dr. Stefan Wastegård, Professor at Stockholm University, Sweden

Chair of Ceremony: Dr. Jukka Käyhkö, Professor at the University of Turku, Finland

This is a joint degree between the University of Iceland and the University of Turku, Finland.

The defence will be streamed live - link here

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. Additionally, datasets of proximal geochemistry of Hekla X, Hekla Y, Hekla Z and Hekla 1845 tephras were produced and published to be used as an aid in establishing more robust correlations between the distal and proximal tephra records. The main outcome of this study is a first outline for a Finnish Holocene tephra framework. The high number of cryptotephra horizons in the framework demonstrates that there is great potential for further cryptotephra studies and utilization of tephrochronology as a dating method in Finland.

About the doctoral candidate: Maarit Kalliokoski was born in Finland in 1978, she moved to Iceland in 1998 and graduated from the University of Iceland with a B.Ph.Isl. – degree in 2002. Since then she has taken turns in living both in Iceland and Finland, working as a translator of Icelandic literature and studying Quaternary geology at the University of Iceland and University of Turku, from which she graduated with BS- degree in 2011 and MS-degree in 2013. She began her doctoral studies at the University of Turku in 2014, and after returning to Iceland in 2017 to work as a Nordvulk fellow at the Institute of Earth Sciences, she started doctoral studies at the University of Iceland towards a joint degree with the University of Turku under a Cotutelle -agreement in 2018

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