Clumped isotopic signatures in land-snail shells revisited: Possible palaeoenvironmental implications

Abstract

It is crucial to understand the clumped isotope compositions (Δ47) of modern land-snail shells and their relationship with environmental parameters for palaeoenvironmental-studies. Previous studies have shown significant variations in relationships between snail-shell Δ47 and ambient air temperature, and the reliability of shell Δ47 in indicating air temperature remains uncertain. This study examined the shell Δ47 from China with mean annual temperatures ranging from 5 to 23 °C and the snail body fluid δ18O estimated from shell δ18O and Δ47-derived temperatures. For all snails studied in this study, site-averaged Δ47 values yield snail calcification temperatures ranging from 25 to 36 °C, which are higher-than-expected in either environmental temperatures or estimated snail activity temperatures. We suggest that snails up-regulate their body temperatures towards their preferred living conditions likely through behaviour or/and physiology adaptation, especially at low temperature environments, which is likely significant than observed previously. Δ47 disequilibrium associated with CO2 degassing during snail calcification is potential but it could possibly be corrected by using an empirical calibration based on natural land-snails. Varying degrees of δ18O enrichment (>8‰) relative to rainwater are observed in snail body fluids, possibly due to variable evaporation during shell calcification. Coupled with model calculations, the relationship between snail body fluid δ18O and rainwater δ18O could be improved by using Δ47-based temperature and δ18O thermometry, if snails prefer to be active at ~90% relative humidity. This study highlights the importance of land-snail shell Δ47 coupled with shell δ18O to indicate temperature and rainwater δ18O at micro-environments scales.