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Radiocarbon Dating Tephra Layers in Britain and Iceland

Published online by Cambridge University Press:  18 July 2016

A. J. Dugmore ,
G. T. Cook ,
J. S. Shore ,
A. J. Newton ,
K. J. Edwards  and
GuÐrún Larsen
A. J. Dugmore
Affiliation:
Department of Geography, University of Edinburgh, Edinburgh, EH8 9XP, Scotland
G. T. Cook
Affiliation:
Scottish Universities Research and Reactor Centre, Scottish Enterprise Technology Park, East Kilbride, G75 0QF, Scotland
J. S. Shore
Affiliation:
Scottish Universities Research and Reactor Centre, Scottish Enterprise Technology Park, East Kilbride, G75 0QF, Scotland
A. J. Newton
Affiliation:
Department of Geography, University of Edinburgh, Edinburgh, EH8 9XP, Scotland
K. J. Edwards
Affiliation:
Department of Archaeology and Pre-History, University of Sheffield, Sheffield, S10 2TN, England
GuÐrún Larsen
Affiliation:
Science Institute, University of Iceland, Reykjavik, IS-101 Iceland
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Abstract

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Layers of volcanic ash, or tephra form widespread chronostratigraphic marker horizons which are important because of their distinctive characteristics and rapid deposition over large areas. Absolute dating of prehistoric layers effectively depends upon 14C analysis. We focus here on Icelandic tephra layers at both proximal and distal sites and consider three strategies to obtain age estimates: 1) the conventional dating of individual profiles; 2) high-precision multisample techniques or “wiggle-matching” using stratigraphic sequences of peat; and 3) a combination of routine analyses from multiple sites. The first approach is illustrated by the dating of a peat profile in Scotland containing tephra from the ad 1510 eruption of Hekla. This produced a 14C age compatible with ad 1510, independently derived by geochemical correlation with historically dated Icelandic deposits. In addition, the ca. 2100 bp date for the Glen Garry tephra in Scotland, determined by a series of dates on a peat profile in Caithness, is supported by its stratigraphic position within 14C dated profiles in Sutherland, and may be applied over a very large area of Scotland. More precise dates for individual tephras may be produced by “wiggle-matching”, although this approach could be biased by changes in peat-bog stratigraphy close to the position of the tephra fall. As appropriate sites for “wiggle-match” exercises may be found only for a few Icelandic tephras, we also consider the results of a spatial approach to 14C dating tephra layers. We combined dates on peat underlying the same layer at several sites to estimate the age of the tephra: 3826 ± 12 bp for the Hekla-4 tephra and 2879 ± 34 bp for the Hekla-3 tephra. This approach is effective in terms of cost, the need for widespread applicability to Icelandic tephra stratigraphy and the production of ages of a useful resolution. We stress the need for accurate identification of tephra deposits without which the conclusions drawn from subsequent 14C dating will be fundamentally flawed.

Type
III. Calibration of the 14C Time Scale
Information
Radiocarbon , Volume 37 , Issue 2 , 1995 , pp. 379 - 388
Copyright
Copyright © the Department of Geosciences, The University of Arizona 

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