CYCLICITY AND ENVIRONMENTAL VARIATION IN THE CARBON AND OXYGEN ISOTOPIC COMPOSITION OF THE MODERN LAND SNAIL CERION ON SAN SALVADOR, BAHAMAS

The land snail, Cerion, has been the focus of several evolutionary and biostratigraphic studies due to its morphologic diversity and abundance in Quaternary eolianites throughout the Caribbean islands. However, the degree to which the shell retains a record of its environment and details of its life cycle remains enigmatic. Carbon and oxygen isotope values obtained during whole shell and sequential ridge analyses of Cerion suggest that a record of diet and environmental conditions on San Salvador, Bahamas, is preserved in shell carbonate.

Carbon isotope values from whole shell analysis of Cerion collected from C₄ vegetation were enriched in ¹³C by an average of 1.0 per mil compared to Cerion collected from C₃ species. A narrow range of d¹³C values observed in Cerion collected from C₃ plants suggests a diet consisting predominantly of C₃ plants while a broader range in d¹³C values observed in Cerion collected from C₄ plants represents a more varied diet. A 0.8 per mil enrichment in d¹⁸O of shell carbonate from the west coast of San Salvador relative to east coast shell carbonate may reflect the incorporation of water vapor derived from isotopically heavy hypersaline lakes occupying the western interior of the island. Sequential ridge analysis of one Cerion shell reveals what appears to be cyclicity in carbonate stable isotopes through ontogeny. Fourier analysis of sequential ridge data revealed 15-ridge cycles in both d¹³C and d¹⁸O through ontogeny that are most likely controlled by changes in diet and variability in rainfall, evaporation and condensation.

Results of stable isotope analyses of modern Cerion suggest that sufficient seasonality exists in tropical regions to be detected in land snail shell carbonate. A similar investigation, using fossil Cerion, has the potential to provide insights into the Pleistocene and early Holocene environments of the Bahamas.