Tephra (volcanic ash) ejected from volcanic eruptions is deposited instantaneously (on geological time scales) creating a layer of equal age in marine and terrestrial sedimentary records, which can then be used as regional dating and synchronisation markers, a technique known as tephrochronology. These deposits are often identified as being undisturbed and primary in nature, but the sampling and analytical methods used may fail to provide the resolution to detect structures that could indicate reworking, and hence affect the technique’s precision and/or validity. Using samples from the RV Pelagia’s expedition 64PE391 we are investigating if the tephra deposits below the Storegga landslide have been disturbed by post-depositional reworking, such as bioturbation, and if so, how this may impact the validity of using the horizon as a dating marker. At BOSCORF, Millie Watts helped us select and prepare the samples from the half-cores into more manageable U-channels for analysis.
Our investigations use X-ray microtomography to image the tephra horizons in 3D and digitally segment the tephra from the host sediment. This gives us a unique insight into the tephra’s geometry that cannot be appreciated by simply studying the core surface or in thin section. We have already started to get extremely promising results from the work which reveal a variety of interesting features in the tephra including biotubration burrows similar to those found below the tephra deposits in the hemipelagic sediment. The other advantage of X-ray microtomography is that after the analysis is complete we can return the U-channels back to BOSCORF for use in other studies.
Some of the preliminary results from this study have just been submitted for publication to the Journal of Microscopy as “Exploring three-dimensional micro-structures of tephra deposits in soft sediments by X-ray microtomography” by Evans, Davies, Wulf, Watts, Talling and Johnston.
For a more light-hearted read about my day out to BOSCORF please see: geologynoteblog.wordpress.com/2017/02/23/trip-to-boscorf