## 4 Conclusions  
For the first time, we applied the CF-LIBS technique to biogenic calcium carbonate to estimate molar concentrations of chemical elements. Our results reported slight differences with molar concentration data derived from ICP-based methodologies, suggesting that algorithms developed to conduct CF-LIBS must be improved in further studies, particularly, to deal with the self-absorption effect that tends to overestimate the concentration of minority elements. Nevertheless, Mg/Ca ratio sequences derived from _P. depressa_ using this novel LIBS technique correctly reflected annual variations in SST, demonstrating that _P. depressa_ incorporates magnesium into its calcium carbonate matrix in dependence with SST in northern Spain. The high correlation between Mg/Ca series and SST at the place where the mollusks grew allowed us to obtain a tentative equation to estimate SST from measured Mg/Ca ratios, although further studies are still required utilizing more samples and improving the CF-LIBS algorithm. Together then, this novel methodology represents a valuable approach for paleoclimate applications in the region, as well as a means of estimating of the season when mollusks from archaeological sites were collected by past humans with implications for the study of human-environment interactions.  
[PERSON] collected the mollusk shells analyzed herein. [PERSON] cut and polished the limpet cross-sections. [PERSON] and [PERSON] performed the measurements by LIBS. [PERSON] developed and applied the CF-LIBS technique to estimate the molar concentrations of magnesium and calcium. All authors ([PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and P.R.) interpreted the data. All authors ([PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and P.R.) wrote and provided comments on the manuscript. All authors have read and agreed to the published version of the manuscript.  
This research was performed as part of the projects HAR2016-75605-R, HAR2017-86262-P and PID2019-107270 RB-C21/AEI/10.13039/501100011033, funded by the Spanish Ministry of Economy and Competitiveness, MINECO. During the development of this research A.G.E was funded by the Basque Country Postdoctoral Program through a postdoctoral grant (POS_2020\(2\)0032). This study has also been supported by the Prehistoric Research Consolidated Group of the Basque Country University (IT-1223-19), funded by the Basque Country Government. P.R. is funded by the Max Planck Society.  
This investigation does not involve research on animals transgenic small laboratory animals, animals transgenic farm animals, non-human primates or animals cloned farm animals. During the development of this study modern mollusc specimens were collected monthly over the course of one complete year. The required authorizations were obtained from Fishing Activity Service of the Cantabrian Government. The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Institutional Review Board (or Ethics Committee) of Fishing Activity Service of the Cantabrian Government.  
Not applicable.  
All of the data reported in the paper are presented in the main text. All Mg/Ca ratio values obtained were included in the Figures 5-7. Sea surface temperatures (SST) at the place where the mollusks grew were previously published [51; 72; 73; 74]. Stable oxygen isotope values used to construct Figure 8 were previously published [72]. The mollusk shells analyzed in this investigation are curated in the Archaeology collection at the University of Cantabria (Santander, Spain). The Matlab implementation of the CF-LIBS algorithm used in this work is openly available at github.com/acobo/CFLIBS.  
We thank the Max Planck Society, University of the Basque Country (UPV/EHU), Grupo de Ingenieria Fotonica (GIF), and University of Cantabria (UC) for support. We are grateful to [PERSON] (GIF) for their scientific support.  
The authors declare no conflict of interest.