SPATIAL PATTERNS OF THE OXYGEN STABLE ISOTOPE COMPOSITION OF LAND SNAIL SHELLS ACROSS NORTH AMERICA

Publication of the first study of the oxygen isotope composition (δ¹⁸O) of land snail shells occurred in 1979. Since then, more than 50 publications have further explored and applied shell δ¹⁸O values in paleoenvironmental studies. Nearly all of the published work has focused on local or regional scale assessments, whereas broad (i.e., hemispheric to global) scale studies remain unexplored. In a recent study, our research group documented that sympatric species of modern small (<10 mm) land snails from North America yielded significantly different δ¹⁸O values (by up to 3‰) in their shells, although isotopic values within phylogenetic groups were similar. This suggests that combining isotopic data from multiple species might not always be appropriate in paleoenvironmental investigations, and researchers should do so only if modern calibration data is available. This poses a problem for broad-scale isotopic studies because encountering the same taxon is virtually impossible along large or contrasting environmental gradients. The present work compiles all published land snail oxygen isotope data in North America, from the Caribbean (18°N) to Alaska (64°N), and evaluates the main climatic controls on shell δ¹⁸O values at a wide latitudinal scale. The results indicate that shell δ¹⁸O values show a strong positive correlation with average annual precipitation δ¹⁸O values and average annual temperature, whereas other parameters such as average annual relative humidity, total precipitation amount, and altitude play only minor roles. Shell δ¹⁸O values increase at the rate of ~0.6‰ for every 1‰ increase in precipitation δ¹⁸O values, and ~0.3‰ per 1°C increase in temperature. These relations hold true when only minute species (< 5 mm) are included, as well as when all snail taxa are considered, regardless of their body size (which ranges from ~1 to ~58 mm in shell length), ecology (herbivores, omnivores, or carnivores), and behavior (variable seasonal active periods and mobility). These results suggest that studies investigating large-spatial scales may be able to combine multiple species for paleoenvironmental reconstructions. More work is needed to determine the scope and magnitude of isotopic differences among sympatric species so that we can better refine these relations over hemispheric scales.