Virtual sampling: Archaeological implications of a new technique for elemental mapping of Mg/Ca ratios in marine mollusc shells

Marine mollusc shells hold significant potential for deciphering past environmental conditions and seasonality of hominin subsistence strategies. While stable oxygen isotope ratio values of shells are currently the gold standard, they have significant drawbacks, such as complex and time-consuming sampling procedures and assumptions on the oxygen isotope composition of seawater in the past. The analysis of shell elemental ratios offers an alternative with minimal sample preparation and no assumptions on water composition. Although elemental ratios have already shown a correlation with seawater temperatures, this relationship is also influenced by other environmental factors and endogenous physiological mechanisms, resulting in noisy ratio profiles that are dependent on the exact measurement path across the growth lines of the shell. This study introduces “virtual sampling” (VS), a novel technique enhancing the analysis of the Mg/Ca ratios of marine mollusc shells measured by Laser-Induced Breakdown Spectroscopy (LIBS). It is based on the automatic detection of the isochronous -growth- lines and its employment for averaging the elemental ratios. This approach mitigates the noise inherent to linear scanning trajectories and improves the estimation accuracy of the elemental ratios. Our investigation focuses on analysing twenty-four modern and six archaeological Patella vulgata Linnaeus, 1758 shells and the effects derived from the application of this virtual sampling versus the traditional techniques. This advancement in elemental analysis provides a more robust basis for seasonal mollusc collection estimations than the linear LIBS scanning and analysis approach, contributing to a better understanding of human lifeways in archaeological studies.