Stable isotope variability and the selection of terrestrial mollusc shell samples for ¹⁴C dating

Assessment of stable isotopic measurements on shell carbonate as a guide to contamination by non-biogenic carbon requires information on variation within individual shells, within a species and between species occupying the same site. The stable carbon and oxygen isotopic composition of living and fossil shells of the land snails Cepaea nemoralis, C. hortensis, Helix pomatia and Pomatias elegans indicates that variations in the δ¹³C of the organic matrix are not significantly influenced by habitat, and that microenvironment and climate override the effects of CO₂ exchange across the body. It follows that, as in marine molluscs, the δ¹³C of unrecrystallised shell carbonate can serve as a useful indicator of errors in ¹⁴C dates caused by recrystallisation or the uptake of old carbon, and that it can be used to screen samples especially if it is determined on CO₂ derived from an entire shell rather than a fragment of shell.