## 3 Results
### Sea Surface Temperature Reconstruction and Paleoclimate Implications  
Reconstructing environmental conditions and climate change during the past, particularly at a seasonal resolution, is key for better understanding the impact of external changes on different aspects of human behavior [4; 5; 14]. Previous sclerochronological studies carried out on mollusks collected from northern Spain have revealed that \(\xi^{18}\)O\({}_{\text{shell}}\) values obtained from _Phorcus lineatus_[51; 73], _P. vulgaris_[74], _P. depressa_[72], and _Mytilus galloprovincialis_[83] can all be considered as effective high-resolution SST recorders. However, to reconstruct SST from \(\xi^{18}\)O\({}_{\text{shell}}\) values, information about \(\xi^{18}\)O\({}_{\text{sw}}\) is also required, the value of which is effectively unknown for the past. Different methodological approaches have been applied in order to overcome this limitation using, for example, the current \(\xi^{18}\)O\({}_{\text{sw}}\) for SST reconstruction during the Roman period [42], or even the Early Holocene period [84]; deciphering \(\rm{\delta^{18}O_{sw}}\) from available salinity datasets [41,85]; or correcting the current \(\rm{\delta^{18}O_{sw}}\) according to known prehistoric sea level changes [86].  
To try and overcome this issue, Mg/Ca ratio analysis has been proposed as a reliable proxy independent of \(\rm{\delta^{18}O_{sw}}\) variations [39,56,68]. However, very few studies have reported a robust dependence between Mg/Ca values and seawater temperatures when carbonate was deposited by the mollusks, and not every species appears to precipitate Mg in equilibrium with environmental surroundings, while the nonquantified effects of other biological and/or environment factors add further dimensions of uncertainty [58, 59, 60, 61,64,65,81]. Therefore, more analyses involving new shell taxa and species collected from coastal areas that were previously not considered are required to better understand trace element precipitation mechanisms. During the last few years, LIBS has emerged as an optimal technique for rapidly investigating if trace element ratios measured on different mollusk species can be used as a climate proxy [57,68,69]. Nevertheless, previous investigations also noticed some limitations of LIBS. These studies showed differences in the Mg/Ca ratio values (from emission line ratios) obtained between specimens, which were caused by inconsistent LIBS experimental conditions and precluded the development of an equation for estimating SST from Mg/Ca values [69]. Here, the LIBS-derived variability was reduced using the CF-LIBS technique, resulting in very similar regression lines between each measured specimen of _P. depresa_ (Figure 9a). This allowed us to tentatively develop the first equation to estimate SST from LIBS-derived Mg/Ca ratios (\(T_{\rm Mg/Ca}\)) considering all maxima, minima, and average values derived from the four shells (Figure 9b):  
\[T_{\rm Mg/Ca}=0.5492\times\rm{Mg/Ca~{}ratio~{}value}-8.5532 \tag{1}\]  
Figure 8: Mg/Ca ratio and \(\rm{\delta^{18}O_{shell}}\) series obtained from (**a**) LAN541, (**b**) LAN545, (**c**) LAN554, and (**d**) LAN559. LIBS derived Mg/Ca ratios were aligned with \(\rm{\delta^{18}O_{shell}}\) series according to major growth lines observed through limpet cross-sections (Figure 3b) and the sclerochronological investigation previously conducted on these same specimens [72].  
Nevertheless, further investigation, increasing the number of shells analyzed and improving the CF-LIBS technique algorithm, is still required in order to obtain a more accurate equation.