Calibration of the carbonate clumped isotope thermometer of land snail shells

Abstract

The application of carbonate clumped isotopes (Δ47) as a temperature proxy has been widely investigated in many types of carbonate materials, including biogenic carbonates such as foraminifera and corals. However, their application to land snails is relatively rare and it remains controversial whether shell carbonate Δ47 is impacted by biologically-driven fractionation. These uncertainties may stem from the lack of precise temperature-controlled calibration samples in previous natural-environment-based studies. In this study we cultured two species of land snail with different environmental tolerances: the thermo-humidiphilous Achatina fulica (A. fulica), and the cold-aridiphilous Cathaica fasciola (C. fasciola), under strictly controlled temperature conditions, covering the growth temperature range of 15–33 °C. The results show that the shell Δ47 values for both A. fulica and C. fasciola had statistically significant negative correlations with the culturing temperature within their respective optimum temperature ranges (A. fulica: 25–33 °C, C. fasciola: 15–25 °C). However, shell Δ47 values for A. fulica at lower temperatures (20 °C in this study) and those for C. fasciola showed a negative deviation from the expected equilibrium values. We attribute the lower-than-equilibrium values to the existence of a vital effect in individual snail species, and more specifically, to the CO2 degassing associated with dehydration/dehydroxylation during carbonate precipitation in the snail's calcifying fluid.