Abstract
The chemical composition of mollusk shells offers information about environmental conditions present during the lifespan of the organism. Shells found in geological deposits and in many archeological sites can help to reconstruct past climatic conditions. For example, a correlation has been found between seawater temperature and the amount of some substituent elements (e.g., magnesium, strontium) in the biogenerated calcium carbonate matrix of the shell, although it is very species-specific. Here we propose the use laser-induced breakdown spectroscopy (LIBS) to estimate Mg/Ca ratios in modern specimens of the common limpet Patella vulgata. An automated setup was used to obtain a sequence of Mg/Ca ratios across a sampling path that could be compared with the seawater temperatures recorded during the organism's lifespan. Results using four shells collected in different months of the year showed a direct relationship between the Mg/Ca ratios and the seawater temperature, although the sequences also revealed small-scale (short-term) variability and an irregular growth rate. Nevertheless, it was possible to infer the season of capture and the minimum and maximum seawater temperatures from the LIBS sequences. This fact, along with the reduction in sampling and measurement time compared with other spectrometric techniques (such as inductively coupled plasma mass spectrometry [ICP-MS]), makes LIBS useful in paleoclimatic studies.
Highlights
The chemical composition of marine mollusk shells offers valuable information about environmental conditions during the organism’s lifespan
Our hypothesis is that physiological mechanisms affecting the magnesium substitution in the biogenesis of the calcium carbonate create small-scale variations not observed by other analytical methods due to the difference in the sampled volume
The feasibility of LaserInduced Breakdown Spectroscopy (LIBS) to measure magnesium incorporation in biogenic calcium carbonate shells has been evaluated. It has been applied for the first time in shells of P. vulgata to obtain sequences of Mg/Ca ratios, that is, the evolution of Mg/Ca ratios throughout the mollusk’s lifespan
Summary
The chemical composition of marine mollusk shells offers valuable information about environmental conditions during the organism’s lifespan. A similar result was found in Pecten maximus but only in the inner calcite layers,[5] while for Mytilus edulis the correlation of the Mg/Ca ratios was very weak for temperature, salinity or age.[6] Several studies have analyzed the shells of the bivalve Arctica islandica due to its exceptional longevity (one specimen found alive in 2006 has the highest reported lifespan among animals: 507 years), but the results using Laser Ablation ICP-MS showed poor or no correlation with seawater temperature and salinity.[7,8] a more recent study based on the same LA-ICP-MS technique[9] that corrects the data for physiological effects due to aging, has found negative correlation of Mg/Ca and Sr/Ca ratios in aragonite layers of this species with seawater temperature These and other studies suggest that the use of Mg/Ca and Sr/Ca ratios as a temperature proxy, especially in bivalve mollusks, is still challenging
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