Search

The results of radiocarbon dating and dendrochronological investigations of timbers (mainly pine and oak) preserved in coastal peats on the Atlantic coast of mid-western Ireland are reported. These are complemented by the results of pollen analytical investigations, including a detailed pollen profile from Lough Atalia, a brackish lake located within present-day Galway city and immediately to the east of the medieval town. The results from the fossil timber investigations show that pine stumps exposed on coastal intertidal peats in mid-western Ireland date mainly to ca. 3000 BC, that is, they are coeval with the so-called ‘pine flushes’ on blanket bog in western Ireland. Many of the oak timbers are considerably older while an oak on peat in Turlin Bay, near Silverstrand, persisted until ca. 2100 BC. This fossil timber record, taken in conjunction with the pollen records, suggests conditions very different to those pertaining at present, including absence of direct marine influence because of relative sea levels (RSLs) that were probably at least 2 m, and possibly up to 4 m, lower than at present. Mid-Holocene conditions were less stormy, and lower precipitation and/or higher evapotranspiration facilitated tree colonisation of peat surfaces. Intertidal peat layers at Salthill show that present-day RSL began to be approached at ca. AD 500. This agrees with the evidence from An Loch Mór, Inis Oírr, which indicates that salinity in this brackish lake began to increase noticeably at/immediately prior to ca. AD 1000. A notable feature of the pollen record from L. Atalia is a yew ( Taxus)-dominated woodland phase of ca. 600 years duration centred on 2550 BC. A major and widespread expansion of yew at about this time is a distinctive feature of mid-Holocene woodlands in western Ireland.

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.