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 chara cteristics and rapid deposition over large areas. Absolute dating of p rehistoric layers effectively depends upon C-14 analysis. We focus her e on Icelandic tephra layers at both proximal and distal sites and con sider three strategies to obtain age estimates: 1) the conventional da ting 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 approa ch is illustrated by the dating of a peat profile in Scotland containi ng tephra from the AD 1510 eruption of Hekla. This produced a C-14 age compatible with AD 1510, independently derived by geochemical correla tion with historically dated Icelandic deposits. In addition, the ca. 2100 sp date for the Glen Garry tephra in Scotland, determined by a se ries of dates on a peat profile in Caithness, is supported by its stra tigraphic position within C-14 dated profiles in Sutherland, and may b e applied over a very large area of Scotland. More precise dates for i ndividual tephras may be produced by ''wiggle-matching'', although thi s approach could be biased by changes in peat-bog stratigraphy close t o the position of the tephra fall. As appropriate sites for ''wiggle-m atch'' exercises may be found only for a few Icelandic tephras, we als o consider the results of a spatial approach to C-14 dating tephra lay ers. We combined dates on peat underlying the same layer at several si tes 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 eff ective in terms of cost, the need for widespread applicability to Icel andic tephra stratigraphy and the production of ages of a useful resol ution. We stress the need for accurate identification of tephra deposi ts without which the conclusions drawn from subsequent C-14 dating wil l be fundamentally flawed.