Confirmation of Mg/Ca ratios as palaeothermometers in Atlantic limpet shells

Abstract

This study provides a reassessment of magnesium to calcium (Mg/Ca) ratios in Atlantic limpet shells to determine past sea surface temperatures (SST). While Patella depressa along the Spanish shoreline and Patella caerulea in the Mediterranean have repeatedly produced reliable correlations between SST and Mg/Ca ratios, this relationship is not the case for other patelloid species. Particularly, Patella vulgata but also Nacella deaurata have been studied using Mg/Ca ratios with mixed or contrary results. In this study, we present elemental maps of these two species as well as Nacella magellanica together with oxygen isotope ratios ($\delta$¹⁸O) that confirm a good relationship with SST. Our dataset also reassesses a specimen which was previously unsuccessful in providing significant correlations between $\delta$¹⁸O values and Mg/Ca ratios. By reassessing these species and including modern and archaeological specimens (n = 12) from three patelloid species (P. vulgata, N. deaurata, and N. magellanica) we further add to the growing set of evidence for the reliable use of Mg/Ca ratios to detect palaeotemperature change. As a result, these species can in the future serve to determine ontogenetic age and season of capture as well as to reveal locations of interest within the growth record (i.e. annual temperature minima and maxima) for targeted $\delta$¹⁸O and clumped isotope analysis.