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XVI INQUA Congress

Paper No. 13
Presentation Time: 1:30 PM-4:30 PM

TOWARDS A HOLOCENE TEPHROCHRONOLOGY FOR SWEDEN


WASTEGÅRD, Stefan1, BERGMAN, Jonas2, BORGMARK, Anders3, HAMMARLUND, Dan2, ROBERTS, Stephen J.4, SCHONING, Kristian3 and WOHLFARTH, Barbara3, (1)Department of Physical Geography and Quaternary Geology, Stockholm Univ, Stockholm, S-10691, Sweden, (2)Department of Geology, Quaternary Geology, Lund University, Tornavägen 13, Lund, S-22363, Sweden, (3)Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm, S-10691, Sweden, (4)Department of Geography, University of Edinburgh, Drummond Street, Edinburgh, EH8 9XP, United Kingdom, stefan.wastegard@geo.su.se

Studies of peat sequences in south and central Sweden have yielded a detailed tephrochronology framework for the middle and late Holocene. Seven tephra layers of Icelandic origin have been detected and geochemically characterised so far. All layers are invisible to the naked eye and extraction techniques involve combustion and acid digestion.

Rhyolitic tephra from Plinian eruptions of the Hekla volcano on SW Iceland dominates and the most significant isochrones are Hekla-4 (c. 4260 cal. yr BP), Hekla-Selsund/Kebister (c. 3750 cal. yr BP), Hekla-3 (c. 3000 cal. yr BP) and Askja AD 1875. These tephras seem to be widespread in Sweden, possibly together with the Hekla-5/Lairg A tephra (c. 7000 cal yr BP). This tephra has been geochemically confirmed in one site, the Klocka bog in central Sweden which so far is the most complete site with five geochemically characterised tephra layers. These well-dated tephras have been used in combination with radiocarbon dates in order to construct age-depth models for cores and sections from ombrotrophic bogs. The Askja-1875 tephra is especially useful for dating recent peat sequences.

Two new late Holocene tephra horizons (the Stömyren tephra, c. 2100 cal .yr BP and the Gullbergby tephra; c. 2700 cal. yr BP) were identified in single sites and are so far less valuable as marker horizons, but are potentially important for the future. The Gullbergby tephra may be an important tephra for correlating peat sequences in Sweden. It is situated at a distinct transition from highly humified peat to less humified peat, which may coincide with the climatic deterioration at the Subboreal/Subatlantic boundary. The source of this tephra is the Torfajökull volcano. The source of the Stömyren tephra is as yet unknown.

Table 1. Geochemically confirmed tephras horizons found in peat in Sweden.

Name of tephra

Age (AD/cal yr. BP)

Source volcano

Askja-1875

AD 1875

Askja

Stömyren

c. 2100 BP

?

Gullbergby

c. 2700 BP

Torfajökull

Hekla-3

c. 3000 BP

Hekla

Kebister/Hekla-S

c. 3750 BP

Hekla

Hekla-4

c. 4260 BP

Hekla

Hekla-5/Lairg A

c. 7000 BP

Hekla

Session No. 41--Booth# 90

T12. Correlation of Ice, Marine, and Terrestrial Sequences using Tephrochronology (Posters)
Saturday, 26 July 2003: 1:30 PM-4:30 PM
, p.147
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