Intra-shell stable isotopes in land snail as proxies of seasonal climate variability: Ontogenetic evidence from cultured and field specimens

Land snail shells preserve stable oxygen and carbon isotope compositions (δ¹⁸O_shell and δ¹³C_shell) that offer valuable seasonal to weather-scale records of terrestrial environmental changes. However, the extent to which ontogenetic processes influence these signals remains insufficiently understood. Here, we investigated intra-shell isotopic variations in giant African land snails from laboratory cultured Achatina fulica and field collected Lissachatina fulica from Panzhihua, China. Laboratory experiments show that adult snail exhibits a δ¹⁸O_shell enrichment of up to 0.8‰, likely driven by internal physiological processes such as biomineralization and metabolism. In addition, δ¹³C_shell show an enrichment of 1.3‰ in subadult and adult shells, potentially associate with increased carbonate ingestion. In natural settings, intra-shell δ¹⁸O_shell variations primarily reflects seasonal fluctuation in precipitation δ¹⁸O, with physiological effects exerting only a minor influence. Although δ¹³C_shell values in wild snails fall within the expected range of C₃ plant-based diets, the potential roles of carbonate ingestion and dietary selectivity should be considered when reconstructing vegetation isotope signatures. These findings establish land snail shells as robust proxies of sub-annual climate variability and offer a modern calibration framework to enhance the use of terrestrial biocarbonates in paleoclimate reconstructions, particularly across monsoonal and moisture-sensitive regions.