## 3 Results  
### Estimation of Mg/Ca Ratio by CF-LIBS  
The elemental Mg/Ca ratio was obtained on a predefined measurement path of the calcite layer for the four shells analyzed (Figure 3b). These ratio sequences are shown in Figure 6a-d. Values on the left-hand side correspond to the shell edge (date of collection) and values on the right-hand side correspond to the area close to the apex, i.e., earlier dates in the mollusk's lifespan. The maximum and minimum Mg/Ca ratio values (LAN541: 59.2 and 39.8 mmol/mol, LAN545: 58.5 and 35.7 mmol/mol, LAN554: 52.7 and 36.8 mmol/mol, LAN559: 55.8, and 31.6 mmol/mol) were relatively consistent between the four shells measured, although LAN554 and LAN559 reported a maximum and a minimum value lower than  
Figure 6: Mg/Ca ratio sequences obtained from (**a**) LAN541, (**b**) LAN545, (**c**) LAN554, and (**d**) LAN559. (**e**) Comparison between Mg/Ca ratio sequences and instrumental SST (\(T_{\rm meas}\)). Red and blue colors indicate warm (i.e., summer) and cold (i.e., winter) temperatures, respectively. those derived from the other shells, respectively. Maximum (59.2 mmol/mol) and minimum (31.6 mmol/mol) Mg/Ca ratio values obtained herein were notably higher than elemental values previously measured on several calcite carbonate mollusk species using ICP-based techniques. [PERSON] et al. [39] and [PERSON] et al. [56] obtained molar concentration values lower than 30 mmol/mol from modern limpets and scallops collected in the southern Iberian Peninsula. Likewise, [PERSON] et al. [65] and [PERSON] et al. [69] analyzed _Patella vulgaris_ limpets from Atlantic Europe, reporting elemental ratios (from ca. 10 to ca. 30 mmol/mol) lower than obtained herein from _P. depressa_ species by LIBS. We attribute this higher concentration of Mg measured by LIBS techniques to a possible species-dependent variability, but also to the CF-LIBS algorithm itself. The results could be affected by the self-absorption of Ca lines, as self-absorption tends to lead to an underestimation of the concentration of major elements in the chemical composition. The current implementation of the algorithm tries to reduce this effect with a careful selection of the emission lines, but the effect is difficult to remove entirely [79]. Another noticeable aspect of the sequences was their high variability, with apparent high-frequency fluctuations on top of the expected yearly sinusoidal variations. Although much research using the LIBS technique has tended to explain general intrinsic variability in LIBS spectra as a consequence of fluctuations in the varying plasma properties of each laser shot [80], we attribute this behavior to the actual composition heterogeneity of the biogenic carbonates on a micrometric scale, as has been suggested elsewhere [58; 61; 69]. As other studies report, the mineralization process in limpets may not only be dependent on seawater temperature, but also on other biological and environmental factors, such as mollusk ontogeny, intra-annual variation in shell growth rates, individual physiological processes, seawater salinity, and/or food supply conditions [60; 64; 81; 82]. To evaluate the reliability of measured Mg/Ca ratios, one limpet (LAN545) cross-section was measured twice along the same path. Although the ablated material was not strictly the same because the second series was measured on material located 20 micrometers deeper than the first series, a high correlation between the two series was expected. In this case, the results obtained were not processed using the SSA algorithm in order to compare actual raw Mg/Ca ratio series. As can be seen in Figure 7, the two LAN545 series measured under the same experimental conditions showed the same trend through time, reporting a high correlation (R\({}^{2}\) = 0.78) between them.