Radiocarbon dating tephra layers in Britain and Iceland

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 (14)C 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 (14)C age compatible with AD 1510, independently derived by geochemical correlation with historically dated Icelandic deposits. In addition, the ca. 2100 sp 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 (14)C 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 (14)C 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 (14)C dating will be fundamentally flawed.