Speakers
Session Three
Large volcanic eruptions disperse volcanic ash thousands of kilometres from the vent. The deposits from these eruptions, termed tephra, form marker layers in sedimentary sequences and ice records. Identifying the tephra layers from particular eruptions allows the sequences that contain them to be correlated, providing a relative chronology. Furthermore, many of the eruptions are dated so identifying the tephra layer can often also provide absolute chronology.
These tephra layers are comprised of volcanic glass (typically >85%) and some crystals. The composition of the volcanic glass varies between volcanoes and different eruption deposits, and so the major, minor and trace element composition serves as fingerprint for a particular eruption (e.g., Lowe 2011). Chemically characterising these tephra layers allows us to correlate sites, which is particularly useful for synchronising and dating palaeoenvironmental and archaeological records (e.g., Lowe et al., 2012). Furthermore, the robust identification of the ash away from the volcanic source provides insight into the magnitude and dynamics of the eruption and helps us to build better records of past volcanism, which is critical for hazard assessments (e.g., Albert et al., 2019).
Accurate and precise glass compositions are required for tephrochronology, and these can be determined using an electron microprobe (EMP) and laser-alation inductively coupled plasma-mass spectrometry (LA-ICPMS). However, glass is easily damaged by an electron beam and to minimise compositional changes it must be analysed using low beam currents and a defocused beam (e.g., Autefage & Couderc, 1980). The major element EMP data are used to normalise the LA-ICPMS data and any problems with accuracy are then transferred into the trace element data. There are a lot of published data that are not accurate, which means that samples need to be reanalysed to confirm correlation. The tephrochronological community is now strongly recommending that reference materials, such as the MPI-DING glasses (Jochum et al., 2006), are analysed during all EMP and LA-ICPMS runs and these secondary standard analyses and are included with the data in publications (Kuehn et al., 2011). This allows for any discrepancies in accuracy to be identified. The issues with accuracy that have been highlighted by the tephrochronological community are likely to be pervasive in published geochemical datasets, which has consequences for large databases and big data initiatives.
School of Archaeology, University of Oxford
Victoria is a specialist in volcanology and the characterisation of volcanic ash layers to date and correlate sedimentary records (tephrochronology) in the School of Archaeology at the University of Oxford. She completed her PhD in Geology at the University of Auckland, New Zealand and moved to the University of Bristol for a 3-year postdoctoral research project in 2006. She joined the School of Archaeology in 2009 to head the tephrochronology research group and manage specialist laboratories to identify and chemically characterise volcanic ash layers. Dr Smith's current research focusses on building detailed records of the composition of explosive volcanism across the globe such that ash layers can be used for far-reaching chronology in both space and time. She is particularly interested in the frequency, size and impact of large eruptions, and is currently working on collaborative projects in Japan, Mexico and Italy.