Seven tephra isochrones in Scotland

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Abstract
This paper reports the presence and geochemical characteristics of seven separate tephras in Scotland that fell in AD 1510, and about 450 BP, 2100 BP, 3600 BP, 3830 BP, 5600 BP and around 6000 BP. The results come from 14 peat bogs spread throughout the Highlands and Islands where 26 tephra deposits have been characterized using over 400 grain-discrete electron-probe micro-analysis. Firstly, tephras are identified and defined by major element characteristics at a reference site. Secondly, tephras are linked between sites on the basis of geochemistry, and dated. Informal names, based on British sites, are suggested for tephras not yet linked to precise sources in Iceland. Despite some apparent variation in colour, all analysed grains in these seven deposits are silicic or intermediate in composition (SiO2 >57%). This phenomenon does not appear to be an artifact of preservation for two reasons. First, small (<40 μm) basic glass shards in peat bogs in northern Iceland are found to have undergone minimal alteration over the last 6000 years; so it is unlikely that basic glass of this age or less in British peat bogs has dissolved since its deposition. Second, tests have shown that the extraction technique used to remove tephra from peat does not significantly alter the mass or particle-size distribution of these glass shards. This study stresses the advantage of using a geographical range of sites to build a regional tephrochro nology in distal areas of volcanic fallout, because there is frequently great local variability in tephra distribution. In the five years since tephras were first identified in northern Britain it has become apparent that tephrochronology can make significant and exciting new contributions to environmental studies throughout this region.