δ18O and δ13C of diagenetic land snail shells from the Pliocene (Zanclean) of Lanzarote, Canary Archipelago: Do they still record some climatic parameters?
Résumé
Fossiliferous bioclastic calcarenites of fluvial-aeolian origin were deposited between 4.3 ± 0.7 Ma and 3.78 ± 0.71 Ma in Lanzarote, Canary Archipelago. Climate was characterized by warm and steppic conditions. The fossil assemblage contains land snail shells that recrystallized into calcite as revealed by Raman spectroscopy. Carbon and oxygen isotope measurements were performed to understand whether or not these isotopic compositions may still reflect some climatic conditions contemporaneous with shell fossilisation and burial. Interpretations have been performed assuming two working hypotheses 1) isotopic compositions still record climatic conditions at the time of snails were living despite diagenesis and 2) isotopic compositions reflect both δ18O and δ13C of soil water as well as soil temperature. Positive correlations are observed between δ18O and δ13C of fossil land snail shells, which are not observed within populations of modern aragonitic land snail shells. This pattern could reflect shell CaCO3 recrystallisation from a CO2-rich aqueous solution that suffered varying rates of evaporation. For the present investigated case, we propose that stable isotope compositions of these diagenetic land snail shells mimic those expected for unaltered snail shells that would still record original climatic conditions. Indeed, δ13C values could result from soil plant decay, which were not significantly different from the δ13C of snail diet mainly based on fresh leaves, even though a bias may result from diet preferences while soil organic matter averages the composition of the vegetation. Oxygen isotope ratios of shells result from a combination of evaporated soil water inherited from meteoric waters and soil temperatures that are closely related to mean air temperatures.
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