## 3 Results
### Estimation of Mg/Ca Ratio by CF-LIBS
Overall, this indicates that, while patterns can be interpreted with confidence, some intrinsic variability is inherent in using the LIBS technique, as well as a local heterogeneity in Mg distribution that is not only related to temperature dependence [58; 61; 69; 70]. Figure 7: (**a**) Two Mg/Ca ratio series obtained from a limpet cross-section measured on limpet LAN545 along the same path and (**b**) the correlation between both series. Mg/Ca Ratio Series vs \(\delta^{18}\)O\({}_{\text{shell}}\) Profiles and Instrumental SST (\(T_{\text{meas}}\))  
The Mg/Ca ratio series obtained showed very well-defined temporal patterns, covering a variable number of seasonal cycles depending on the age of the shell (Figure 6a-d). A cycle refers to that part of the series between one minimum or one maximum value and the next one. Each cycle is therefore composed of a maximum, a minimum, and a variable number of intermediate values. A variable number of cycles were documented for each specimen. LAN541, LAN554, and LAN559 exhibited elemental fluctuations corresponding to approximately one cycle, while LAN545 covered a time span longer than two cycles. The number of cycles reported by the elemental series for each limpet was very consistent with the actual number of years of the life-span of these specimens, which was previously decoded through a detailed sclerochronological investigation carried out on these four same specimens combining a stable oxygen isotope analysis and a study of the growth patterns on every shell [72]. According to the results obtained from the previous trace element investigations carried out on biogenic carbonates [56; 68], and especially those conducted on limpet species [39; 57; 58; 69], higher and lower Mg/Ca ratio values represent warmer and colder SST, respectively, and each cycle corresponds to a complete year. This approach was also confirmed by the results derived from the sclerochronological study conducted on these four shells [72], since the cross-section areas where the minima and maxima Mg/Ca ratio values were located coincided very well with those limpet portions deposited when coldest and warmest seawater temperatures were reached, respectively. A comparison between the obtained Mg/Ca ratio series and instrumentally measured SST (\(T_{\text{meas}}\)) at the place where the mollusks grew (Figure 6e) demonstrated a strong agreement between the two datasets through time, covering the expected number of summer and winter seasons through each mollusk's lifespan. Finally, the Mg/Ca ratio series obtained herein were also compared with \(\delta^{18}\)O\({}_{\text{shell}}\) values previously published for these same specimens [72]. The results reported a very high correlation between both profiles (R\({}^{2}\) = 0.78-0.87) (Figure 8). Therefore, LIBS-derived Mg/Ca ratio series from _P.depressa_ clearly reported SST variations during the lifespan of the mollusk, covering a variable time span between one year (LAN559, Figure 6d) and almost three annual cycles (LAN545, Figure 6b). In order to quantify the seawater temperature dependence of Mg incorporation into the carbonate matrix, maxima, minima, and average Mg/Ca ratios obtained for each annual cycle, and maxima, minima and average \(T_{\text{meas}}\) for each year were compared. Linear regression analysis showed very high correlations in each of the four shells (R\({}^{2}\) = 0.91-0.95) (Figure 9a), even when all four shells were considered together in the same analysis (R\({}^{2}\) = 0.86) (Figure 9b). Results obtained herein, therefore, for the first time, show that the _P.depressa_ mollusk species incorporates magnesium into its calcium carbonate matrix in dependence with SST in northern Spain. Furthermore, the LIBS-derived Mg/Ca ratio series have enough resolution to correctly reflect the general evolution of SST variations on a year-round basis, with clearly distinguishable warmer and colder periods.