A tephrostratigraphic investigation of the continuously varved Holocene Boreal lake Nautajärvi, Finland, provides precise age estimate for Lairg A/Hekla 5 eruption

Abstract

Numerous cryptotephra layers originating from Icelandic volcanoes and further afield have reached Northern Europe during the Holocene. Refining the precise timing and the relative frequency of local and distal eruptions requires well‐resolved continuous sediment archives. Lake Nautajärvi is located in central‐southern Finland (61°48′ N, 24°41′ E), and contains a continuous sequence of 9898 varves. Nautajärvi was isolated from the Baltic Sea basin at the Lake Ancylus phase ~9600 varve yr BP. We present a tephrostratigraphic record of Nautajärvi for targeted intervals within the Holocene, aiming to identify tephra from the largest eruptions, to enable an alignment with other varved lakes in Northern Europe containing tephra layers, like Diss Mere, Tiefer See and Meerfelder Maar. Geochemical analyses were produced for 26 peaks in tephra concentration, with five cryptotephra deposits corresponding to known eruptions from the mid and late Holocene. The far‐travelled KS 2 eruption from Ksudach caldera in Kamchatka is identified, enabling an alignment to Greenland, which is strengthened by the identification of the Hekla 4 tephra in Nautajärvi. Close to the KS 2 deposition in Nautajärvi, a Hekla‐type eruption correlating to the Lairg A/Hekla 5 is also identified—macroscale counting of the varves between the KS 2 and Lairg A layers enables a constrained age estimate (7001 ± 44 Cal yr BP) of the Lairg A eruption to be made. The widespread Glen Garry tephra was also identified within the Nautajärvi sequence, with an age estimate of 2088 ± 21 varve yr BP. Finally, there is also a tentative correlation to the Suduroy tephra, with an older age estimate than from the sites in which it was originally identified (8459 ± 84 varve yr BP). The remaining layers correspond to eruptions from more distal sources—several chemistries may correlate to mid‐Holocene eruptions from North America (Mount Rainier, Aniakchak, Katmai), with one shard tentatively associated with Azorean chemistry. This record provides well‐constrained age estimates for two key Holocene tephras (Glen Garry, Lairg A) and highlights the potential for far‐travelled tephras from North America and Kamchatka to reach Northern Europe, emphasising the benefits of further tephra studies in Scandinavia.

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