Oxygen and carbon stable isotopes of modern land snail shells as environmental indicators from a low-latitude oceanic island

Abstract

Land snails provide a unique opportunity to study terrestrial paleoenvironments because their shells, which are generally highly abundant and well-preserved in the fossil record, contain a temporal record of environmental change in the form of isotope codes. To evaluate the utility of this approach for a low-latitude oceanic setting, 207 modern shells of 18 species of land snail were analyzed for their oxygen and carbon isotope composition along a north and south facing altitudinal gradient (10–2160 m a.s.l.) in Tenerife Island (∼28°N) of the Canary Archipelago. Shells collected at each locality showed a relatively large range in isotope composition which was greater along the south facing transect (drier and hotter), suggesting that the variance in shell isotope values may be related to water-stress. Although pooled isotope values did not generally show strong relationships with environmental variables (i.e., altitude, temperature and precipitation), mean isotope values were strongly associated with some climatic factors when grouped by site. The mean δ 18O value of the shell (δ 18O shell) by site displayed a negative correlation with elevation, which is consistent with the positive relationship observed between temperature and the δ 18O value of rain (δ 18O rain). Calculated δ 18O values of the snail body water (δ 18O body) derived from observed temperatures and δ 18O shell values (using the equation of Grossman and Ku [Grossman E. L. and Ku T. L. (1986) Oxygen and carbon isotope fractionation in biogenic aragonite. Chem. Geol. (Isotope Geosci. Sec.) 59, 59–74]) displayed a trend with respect to altitude that was similar to measured and hypothetical δ 18O values for local rain water. The calculated δ 18O body values from the shell declined 0.17‰ (VSMOW) per 100 m, which is consistent with the “altitude effect” observed for tropical rains in Western Africa, and it correlated negatively with rainfall amount. Accordingly, lower δ 18O shell values indicate lower temperatures, lower δ 18O rain values and possibly, higher rainfall totals. A positive correlation between the mean δ 13C values of shells (δ 13C shell) and plants by site suggests that shells potentially record information about the surrounding vegetation. The δ 13C shell values varied between −15.7 and −0.6‰ (VPDB), indicating that snails consumed C 3 and C 4/CAM plants, where more negative δ 13C shell values probably reflects the preferential consumption of C 3 plants which are favored under wetter conditions. Individuals with more positive δ 13C shell values consumed a larger percentage of C 4 plants (other potential factors such as carbonate ingestion or atmospheric CO 2 contribution were unlikely) that were more common at lower elevations of the hotter and drier south facing transect. The relatively wide range of shell isotope values within a single site requires the analysis of numerous shells for meaningful paleoclimatic studies. Although small differences were observed in isotope composition among snail species collected at a single sampling site, they were not significant, suggesting that isotope signatures extracted from multi-taxa snail data sets may be used to infer environmental conditions over a broad range of habitats.