24 Jun 2026 - Amy
Once again, I have failed in my very important mission of actively maintaining a blog. In my defence, I have been away for three weeks this last month, and I am in the middle of moving house, which I have (ridiculously) decided to do amidst a heatwave. The last month has been, however, unbelievably intellectually stimulating and rewarding, so I’m excited to start updating you on that. As I have been on three separate trips, I think I will try and cover them, and my subsequent thoughts, across the next three months, assuming that I learn to keep to a schedule.
So, at the end of May, I went back to Cambridge to use the Electron Probe Micro Analyser (henceforth EPMA) in the Department of Earth Science. The machine, run by the unbelievably excellent Dr Iris Buisman, is one I used during my PhD, so it was nice to return to my old haunts and fire some electron beams at some glass shards. And I guess I should explain why…
Tephrochronology often begins in the field and the laboratory - a subtle hint of ash in an outcrop/section or a core, or a barely-there peak in glass shard counts. The real work of turning that horizon into a robust time marker, however, happens at the microprobe. EPMA provides the geochemical “fingerprint” that allows us to correlate tephra layers across sites, regions, and archives with confidence.
Put simply, EPMA measures the elemental composition of individual volcanic glass shards. These shards, typically only tens of microns across, retain the melt composition of the magma at the moment of eruption. Because many eruptions produce compositionally distinctive glasses, this chemistry can be used to link dispersed tephra horizons back to specific volcanic events. In practice, this is what transforms a cryptic ash layer into a precise chronological tie-point.
The process is technically demanding but conceptually straightforward. After isolating shards (often through density separation and careful picking under a microscope, an unfathomably tedious process) they are mounted in resin, polished to expose internal surfaces, and coated to ensure conductivity. Under the microprobe, a focused electron beam excites atoms within the glass, producing characteristic X-rays. Measuring these X-rays allows us to quantify the abundance of major and minor elements such as Si, Al, Fe, Ca, Na, K, and Ti.
What makes EPMA particularly valuable for tephrochronology is its scale and precision. We analyse individual shards rather than bulk samples, which means we can detect mixed populations, assess compositional variability, and avoid the averaging effects that obscure correlations. This is especially important in cryptotephra work, where shards are sparse and potentially derived from multiple sources. Careful analytical protocols—low beam currents, defocused beams, and short counting times—are essential to minimise alkali migration and preserve data quality. Even then, the data require considered filtering and normalisation.
The resulting geochemical dataset is the basis for correlation. In many cases, simple bivariate plots (for example, FeO versus CaO, or K2O versus TiO2) are sufficient to distinguish between candidate eruptions. Where compositions overlap more closely, multivariate approaches and comparison with well-curated reference datasets become necessary. Here, the strength of tephrochronology lies not just in the analysis itself, but in the collective effort to build regional and global geochemical frameworks. Databases of well-characterised tephras allow new analyses to be placed within a broader context, strengthening or, occasionally, challenging existing correlations.
Nothing is perfect in science, however, so it is worth acknowledging the limits of EPMA. Major element chemistry alone does not always uniquely identify an eruption, particularly in volcanic systems that produce compositionally similar magmas over time. Analytical uncertainties, inter-laboratory offsets, and natural glass heterogeneity all introduce ambiguity. For this reason, EMPA is most powerful when combined with additional lines of evidence: stratigraphic position, shard morphology, grain size distributions, and, where possible, trace element or isotopic data. In many projects, EPMA provides the essential first pass—rapid, relatively accessible, and highly informative—before more specialised analyses are deployed.
In tephrochronological applications, these geochemical fingerprints underpin a wide range of scientific questions. They allow synchronisation of marine, terrestrial, and ice-core records; testing of leads and lags in climate responses; and the construction of independent age models in settings where other dating methods are limited. The ability to correlate a thin layer of ash across hundreds or thousands of kilometres remains one of the most powerful tools at our disposal as Quaternary researchers.
There is also a practical dimension. Standardisation (both in analytical conditions and in data sharing) is critical here, particularly for projects that aim to integrate results from multiple laboratories. So much work has been done to standardise these approaches, and I’m excited to begin applying these in my own work. I have, because of the nature of PostDoc life, been out of the tephra game for a couple of years, so need to catch up.
Ultimately, electron microprobe analysis is more than an analytical step. It is the point at which microscopic material becomes chronologically meaningful. When done carefully, EPMA allows us to move from “this looks like ash” to “this is the same eruption observed elsewhere, at a known time,” which is ultimately what tephrochronology is for.
Anyway, thanks for having me, Iris, hopefully I’ll be back soon!
I’ll see you next month.