7 TEPHRA ISOCHRONES IN SCOTLAND

This paper reports the presence and geochemical characteristics of sev en 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 c ome from 14 peat bogs spread throughout the Highlands and Islands wher e 26 tephra deposits have been characterized using over 400 grain-disc rete electron-probe micro-analysis. Firstly, tephras are identified an d defined by major element characteristics at a reference site. Second ly, tephras are linked between sites on the basis of geochemistry, and dated. Informal names, based on British sites, are suggested for teph ras not yet linked to precise sources in Iceland. Despite some apparen t variation in colour, all analysed grains in these seven deposits are silicic or intermediate in composition (SiO2 >57%). This phenomenon d oes not appear to be an artifact of preservation for two reasons. Firs t, small (<40 mu m) basic glass shards in peat bogs in northern Icelan d are found to have undergone minimal alteration over the last 6000 ye ars; so it is unlikely that basic glass of this age or less in British peat bogs has dissolved since its deposition. Second, tests have show n that the extraction technique used to remove tephra from peat does n ot significantly alter the mass or particle-size distribution of these glass shards. This study stresses the advantage of using a geographic al range of sites to build a regional tephrochronology in distal areas of volcanic fallout, because there is frequently great local variabil ity in tephra distribution. In the five years since tephras were first identified in northern Britain it has become apparent that tephrochro nology can make significant and exciting new contributions to environm ental studies throughout this region.