Tephrochronology

A key requirement for the success of HoStIr is good age control. This is the case with any palaeoenvironmental project, where a timeframe must be constructed for the stratigraphy in order to know when the events inferred by various environmental proxies occurred.

One way to do this is to use tephrochronology, with layers of volcanic ash (tephras) providing time-lines in the stratigraphy. The layers are often far-travelled and are deposited within a few days following an eruption, which is virtually instantaneous in palaeoenvironmental or archaeological contexts. The microscopic form of this ash (cryptotephra) is particularly useful as it is the farthest travelled, being deposited and preserved in sediments thousands of kilometres from source. These layers contain glass particles smaller than 100 µm, but still retain the distinct chemical signature or ‘fingerprint’ of the source volcano, allowing reliable identification of the layer. Furthermore, many of these layers have been used in previous studies where they have been dated, often by radiocarbon dating. Therefore, their subsequent detection elsewhere comes complete with this date – effectively, free radiocarbon dates falling from the sky.

The technique works particularly well in peat sediments and Ireland abounds with peat bogs, plus a plentiful and regular supply of tephra layers over the millennia courtesy of the many volcanoes of Iceland just over 1000 km to the northwest. The HoStIr project will take full advantage of this technique to correlate between transect cores and sites throughout Ireland, and to construct age models to date the various proxies for storminess.

By Sean Pyne-O'Donnell