Abstract
Measurement of the elemental composition of shells is increasingly emerging as an avenue for obtaining high-resolution insights into paleoclimate and past seasonality. Several studies have shown significant correlations between Mg/Ca ratios measured on shell carbonate and the sea surface temperature (SST) within which this carbonate was precipitated. However, other investigations have reported large variability in this relationship between species. Therefore, further studies, including taxa previously not considered are still required in order to validate these new species as suitable climate proxies. Here, we measured Mg/Ca ratios for limpet Patella depressa Pennant, 1777 samples live-collected in northern Spain for the first time. The elemental ratio was measured using laser-induced breakdown spectroscopy (LIBS), a technique that significantly decreases the time required for sample preparation and increases the number of shells that can be analyzed. In this study, calibration-free LIBS (CF-LIBS) methods were applied to estimate molar concentrations of chemical elements on biogenic calcium carbonate. The Mg/Ca ratio evolution along the shell growth axis was compared with stable oxygen isotope (δ18O) profiles obtained from these same limpets and the SST at the place where the mollusk grew to determine if the sequences obtained correctly reflected environmental conditions during the life-span of the mollusk. The results showed a significant correlation between Mg/Ca ratio series and both δ18O profiles and SST, highlighting the paleoenvironmental and archaeological potential of LIBS analyses on this mollusk species that is frequently found in archaeological contexts in the western Europe.
Highlights
There has long been a keen theoretical and methodological interest in archaeology at elucidating the impacts of climatic and environmental change on hominin behavior, settlement, and culture in different parts of the world at different points of time [1,2,3,4,5], and how this might be mobilized to inform challenges facing our species in the 21st century [6,7]
Stable oxygen isotope ratios derived from marine mollusk shells (δ18Oshell) are powerful recorders of the seasonal sea surface temperature (SST) variations experienced during the precipitation of carbonate year-round in the past [39,41,42,43]
Our results reported slight differences with molar concentration data derived from ICP-based methodologies, suggesting that algorithms developed to conduct CF-laser-induced breakdown spectroscopy (LIBS) must be improved in further studies, to deal with the self-absorption effect that tends to overestimate the concentration of minority elements
Summary
There has long been a keen theoretical and methodological interest in archaeology at elucidating the impacts of climatic and environmental change on hominin behavior, settlement, and culture in different parts of the world at different points of time [1,2,3,4,5], and how this might be mobilized to inform challenges facing our species in the 21st century [6,7]. Previous studies on modern marine mollusk shells have shown a high correlation between δ18Oshell values and SST [48,49,50,51], oxygen isotope fractionation is influenced by oxygen isotope composition of the seawater (δ18Osw) [20,52] This represents a significant limitation for paleoreconstructions where δ18Osw values are unknown, while the cost- and time-consuming nature of serial δ18Oshell sampling can be prohibitive. We measured, for the first time, Mg/Ca ratios of limpet Patella depressa Pennant, 1777 samples live-collected in northern Spain This mollusk species was consumed in high numbers by Mesolithic humans during the Early Holocene period, along the Atlantic coast of Europe, representing an important percentage of all shell taxa recovered from shell middens excavated in northern Spain and France [66,67].
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