Implications of the apparent ¹⁴C age of cultured Achatina fulica and the spatial features of ¹⁴C ages among modern land snail shells in China

Abstract

It has long been known that many terrestrial snail shells are subject to the so-called “limestone effect”. However, the magnitude and impact of this effect on the δ¹³C values of snail shells and soft body tissue (δ¹³C_(shell) and δ¹³C_(body)) are still unknown. In this study, Achatina fulica snails were hatched and cultured under controlled conditions and supplied with ¹⁴C-free calcium carbonate and lettuce (a C₃ plant) as a calcium source and food supply, respectively. The snail shells and soft body tissues were sampled monthly for 5 months, and δ¹³C and radiocarbon dating analyses were conducted to trace carbon source variations at different growth stages. Our results show that the apparent ¹⁴C age of the shells and the corresponding δ¹³C_(shell) values increased progressively from the 1st to 3rd months and subsequently decreased during the 4th and 5th months. The results demonstrate for the first time that carbonate ingestion has a straightforward influence on the ¹⁴C ages and δ¹³C values of shells and that the “limestone effect” changes over the lifetime of snails. Moreover, the apparent ¹⁴C ages of snails sampled in the field are older than modern ages when taken from the northern carbonate region and relatively young or the same as modern ages when collected from ferralic soils. This further supports our findings from cultured snails and suggests that snail shells are good archives for radiocarbon dating purposes in southern China. Although the δ¹³C values of the soft body tissues of cultured snails vary synchronously with those of the shell, this seemingly common phenomenon cannot be easily explained by carbonate ingestion, as the snail bodies are of more recent origin than the shells. Atmospheric CO₂ may be another candidate explaining the synchronicity between δ¹³Cshell and δ¹³Cbody, as well as the F ¹⁴C variations of snail bodies. However, it is still unclear how snails differentially incorporate carbon from different sources (e.g., carbonate and atmospheric CO₂) at different growth stages, since carbon from different sources is combined in the bicarbonate pool in the hemolymph before being used by snails.