Calibrating the limpet

Our new study delivers the most extensive calibration to date of Patella caerulea (the Mediterranean limpet) as a recorder of sea surface temperatures (SSTs) across the Central and Eastern Mediterranean. The study combined laser-induced breakdown spectroscopy (LIBS) for measuring Mg/Ca ratios with isotope ratio mass spectrometry (IRMS) to analyse oxygen isotope values (δ¹⁸O). By analysing 131 modern shells from 22 sites, we explored the relationship between magnesium-to-calcium (Mg/Ca) ratios and oxygen isotope values as palaeotemperature proxies.

Results revealed a strong correlation (R² = 0.89) between Mg/Ca ratios and δ¹⁸O-based sea surface temperature estimates. 400 elemental (Mg/Ca) maps and 132 line scans were produced from the shells, which showed clear seasonal cyclicity patterns in over 90% of samples.

Oxygen isotope measurements from shells and water samples provided reliable estimates of sea surface temperatures (SSTs) for the collection seasons, once adjusted with empirical corrections (offsets). However, as these offsets differ by season and location, site-specific calibrations are essential when applying palaeotemperature equations in archaeological contexts.

This large comparative dataset establishes P. caerulea as a robust palaeotemperature archive, offering new opportunities for refining palaeoclimate reconstructions in Mediterranean archaeological contexts. The study highlights the advantages of combining rapid elemental (Mg/Ca) mapping with (δ¹⁸O) analysis—allowing researchers to detect seasonal patterns and evaluate shell growth before conducting isotope analyses. These insights provide a valuable foundation for future palaeoclimate research, improving the precision of past temperature reconstructions and enhancing our understanding of historical climate variability in the Mediterranean region.

Acknowledgements The research was supported by the German Research Foundation (DFG) through the Emmy Noether Program. The team gratefully acknowledges the Applied Photonics Lab at the Institute of Electronic Structure and Laser (FORTH, Greece) for their assistance with LIBS analyses, and Dr. Hubert Vonhof at the Max Planck Institute for Chemistry for conducting water sample analyses. Fieldwork in the National Marine Park of Alonissos–Northern Sporades was made possible through a permit from the Greek Ministry of Environment and Energy, with on-site support from Stefanos Paraskevopoulos, Spyros Iossifidis, and Angeliki Kita.

Reference: Theodoraki, D., Mannino, M. A., Prendergast, A. L., Schöne, B. R., & Hausmann, N. (2025). Multi-regional calibration of Patella caerulea as a palaeothermometer using oxygen isotope values and Mg/Ca ratios. The Holocene, 0(0). https://doi.org/10.1177/09596836251378034