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

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 (similar to 28 degrees 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 delta O-18 value of the shell (delta O-18(shell)) by site displayed a negative correlation with elevation, which is consistent with the positive relationship observed between temperature and the delta O-18 value of rain (delta O-18(rain)). Calculated delta O-18 values of the snail body water (delta O-18(body)) derived from observed temperatures and delta O-18(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. Chen?. Geol. (Isotope Geosci. Sec.) 59, 59-74]) displayed a trend with respect to altitude that was similar to measured and hypothetical delta O-18 values for local rain water. The calculated delta O-18(body) values from the shell declined 0.17 parts per thousand (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 delta O-18(shell) values indicate lower temperatures, lower delta O-18(rain) values and possibly, higher rainfall totals. A positive correlation between the mean delta C-13 values of shells (delta C-13(shell)) and plants by site suggests that shells potentially record information about the surrounding vegetation. The delta C-13(shell) values varied between -15.7 and -0.67 parts per thousand (VPDB), indicating that snails consumed C-3 and C-4/CAM plants, where more negative delta C-13(shell) values probably reflects the preferential consumption of C-3 plants which are favored under wetter conditions. Individuals with more positive delta O-13(shell) values consumed a larger percentage of C-4 plants (other potential factors such as carbonate ingestion or atmospheric CO2 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. (c) 2009 Elsevier Ltd. All rights reserved.