Tephrostratigraphy of An Loch Mór, Inis Oírr, western Ireland: implications for Holocene tephrochronology in the northeastern Atlantic region

Abstract

Twelve definable cryptotephra layers younger than c. 8600cal. BP are reported from lacustrine core material taken from An Loch Mór, Inis Oírr, Aran Islands, western Ireland. The geochemistry of these shard layers, which represent more Holocene tephras than previously geochemically characterized from any European site outside the proximal volcanic influence of Italy or Iceland, is presented. Of these tephras, four may correlate with known Iceland-derived tephra horizons (Vo 1477, HI, Lairg A and H5); one appears to have two possible named correlatives (Hoy;‘Lairg B’ at Sluggan Bog, N Ireland); another has a plausible correlative (AD 860A); but two others represent Icelandic tephras apparently not previously reported on Iceland itself, including a tephra that has geochemistry very similar to Hl (AD 1104) but which dates to c. cal. AD 840. Four tephras that date from the period c. AD 40-1400 have a distinct geochemical signature that relates to Jan Mayen, 750km to the northeast of Iceland and some 1800km distant from An Loch Mór. The results contribute substantially to the chronology of the sediments from An Loch Mór and, at the international level, to the tephrostratigraphy of the northeast Atlantic region. The well-attested Icelandic H4 tephra (from a Hekla eruption of c. 4260cal. BP) was not recorded; it is suggested that An Loch Mór lies south of its recordable distribution in this part of the northeast Atlantic region. Taken together with work at other sites, the findings point to a wealth of potential tephra isochrons in Holocene deposits of the northeast Atlantic seaboard. It is clear also that calcareous lake sediments can be as fruitful a source of tephras as peats. Potential problems relating to dating and geochemical fingerprinting of tephra layers in individual sites are highlighted with reference to the Lairg tephras as recorded in Scotland and Northern Ireland, and to the Hoy tephra that has been described from Orkney.