Clumped isotopic compositions of cultured and natural land-snail shells and their implications

Abstract

Land snails are widely distributed animals whose clumped isotope (∆₄₇) in shell carbonates can serve as a proxy for temperature in terrestrial environments. However, it is unclear whether the ∆₄₇ values of snail shells are affected by biological processes known as “vital effects.” Here, we report ∆₄₇ values of whole Achatina fulica snail shells cultured at different temperatures at different growth ages (samples collected monthly). In general, the mean ∆₄₇ values of the snail shells decrease with higher cultivation temperature, which indicates they have potential use in reconstructing temperature changes. However, the mean reconstructed ∆₄₇ temperatures (T₄₇) using the calibrations from Petersen et al. (2019), Bonifacie et al. (2017), and Kelson et al. (2017) are ~6 °C higher than the actual growth temperatures. Interestingly, this overestimation in T₄₇ disappears when applying Zhai et al.'s (2019) calibration for the Bradybaena snail. This calibration also yields good agreement between the T₄₇ data of all field-collected snails and their growing season temperatures. The above-mentioned phenomenon indicates the existence of the vital effect in land-snail shells. The variations in ∆₄₇ with snail age and the correlations between ∆₄₇, δ¹⁸O, and δ¹³C values also indicate the potential influence of vital effects. Finally, we suggest that the vital effect may be explained by kinetic fractionation through CO₂ degassing associated with dehydration/dehydroxylation during the formation of snail shells.