What’s in a shard? Potential insights from sparse cryptotephra deposits

Cryptotephra deposits have long been used as an age equivalent geochronological method on the basis that their glass geochemistry enables linkages to the eruptions that produced them. Their analysis in palaeoenvironmental research has further contributed to volcanic histories, and to investigating the environmental and societal impacts of past eruptions. Cryptotephra in highly resolved polar ice cores has been particularly useful for aiding the understanding of the relationship between volcanic eruptions and past climate responses, while at the same time affording precise ages for prehistoric eruptions. Recent efforts to expand the usefulness of cryptotephra analysis include inverse ash dispersion modelling to infer eruption source parameters as a means of refining volcano-climate modelling (Plunkett et al. 2022). Cryptotephra shard concentrations exhibit high spatial variability, however, and can be limited by the distance between the source and the deposition site, especially if latitudinal distances are great, as well as meteorological conditions that govern their transport and deposition. Low shard concentrations, sometimes comprising individual grains, present challenges for interpretation, not least by limiting the potential for robust geochemical datasets.<br/><br/>This paper considers to what extent data obtainable from sparse cryptotephra can be used to understand past eruptions. Research on macro-tephra deposits has shown that shard size and shape reflect eruption style and impact ash dispersal, but suggest that large sample sizes and complex imaging analysis are needed to make statistically robust inferences (Saxby et al. 2020; Benet et al. 2024). Here we collate data from cryptotephra deposits from historic eruptions, including their geochemical characterisation, grain size, shape and texture, and consider their relationship to known eruption characteristics. We compare the information obtained from the first sighted shard in a sample to progressively larger datasets from the same sample to evaluate the extent to which reliable inferences might be drawn from a restricted number of shards. We conclude with recommendations for data recording that can be effectively implemented during routine shard counting on a standard light microscope.<br/><br/> References<br/>Benet D et al. (2021) Bull Volcanol 86: 9. https://doi.org/10.1007/s00445-023-01695-4<br/>Plunkett G et al. (2022) Climate Past 18: 45–65. https://doi.org/10.5194/cp-18-45-2022<br/>Saxby J et al. (2020) J Quat Sci 35: 175–185.<br/>