Extending the known distributions of micro-tephra layers of the last glacial-interglacial transition age in Europe
A number of terrestrial stratigraphic sequences spanning the last glacial-interglacial
transition from sites in Britain and continental Europe have been examined to assess the
potential of tephrochronology as a precise correlation tool, and to contribute towards the
construction of a tephrochronological framework for Europe. The distributions of four
Icelandic tephras have been extended in Europe through the detection of micro-tephra
horizons. These results include (i) the discovery of the Vedde Ash in the north-eastern
Netherlands, (ii) the detection of the Borrobol Tephra and of the relatively unknown
Askja 10.0 14Cka BP Tephra in south-eastern Sweden; and (iii) the discovery of the
basaltic component of the Vedde Ash for the first time in two lake sequences in Britain,
one being a visible horizon (I cm thick) and the other being a micro-tephra horizon.
Potential areas in Europe that may have received tephras from more than one volcanic
province are proposed based on the revised distributions of these tephras. Geochemical
analyses by electron microprobe also point to the discovery of at least 4 previously
undetected tephras, two of which are thought to have originated from the Icelandic
volcanic province, while two are of unknown origin.
Detailed investigations of one terrestrial site in the British Isles suggest that tephra
horizons can be unevenly developed within lake basins, supporting the view that lake
basin and catchment processes, as well as meteorological factors prevalent during tephra
dispersal, influence the distribution and accumulation of tephra horizons.
Methodological developments in detection techniques have enabled the extraction of a
micro-tephra layer of basaltic geochemistry from minerogenic lake sediments, and
experimentation with the determination of trace element composition of tephra samples
by solution Inductively Coupled Plasma-Mass Spectrometry (lCP-MS) has highlighted
a number of limitations associated with the analysis of very small samples.
Recommendations for improved application of this technique in order to develop it as a
diagnostic tool for the geochemical fingerprinting of tephra horizons are suggested.
Results are also presented of investigations of micro-tephra horizons within the
NordGRIP ice core (north of Greenland Summit). A possible total of six new tephra
horizons have been detected. The geochemical results suggest that these shards may
have originated from the Grimsvotn volcanic system (Iceland), although close similarity
to the Saksunarvatn Ash, which has also been identified in this project, suggests the
possibility of contamination within the filtration system employed. Recommendations
for improvements to tephrochronological research in future work on the NordGRIP ice
core are outlined.