STABLE ISOTOPE COMPOSITION OF MODERN LAND SNAILS FROM CANARY ISLANDS

Stable isotopes extracted from land snails offer a useful tool for estimating paleoenvironments and paleoclimates. However, isotope studies of land snails from low latitudes have been scarce. This study focuses on present-day helicid gastropods from the eastern Canary Islands (27º 37' and 29º 25' N) and aims to enhance our understanding of the interpretative value of stable isotopes extracted from fossil snails. The d18O, d13C, and dD data were collected from nine sites and included 17 samples of aragonite shells and 17 samples of body tissue of the gastropod Theba geminata, a species abundant in local Pleistocene sediments. Bioclastic sediments (n=10), plants (n=69), and rain water (n=7) have also been analyzed. The average isotopic compositions of the rain water are -5‰ for dD and -2 ‰ for d18O, ranging from -11 to 2‰ and -2.6 to 0.7‰ (V-SMOW), respectively. Water samples were not biased by evaporation, as they plot linearly along the Meteoric Water Line. The vegetation has a heterogeneous composition of C3-C4 type plants (d13C between -12.99‰ and -28.95‰). Of 24 species, at least 5 follow a C4 metabolism, 15 a C3 metabolism, and 4 could be considered CAM type plants. Modern aragonite shells show d18O range from -0.25‰ to 2.49‰ (V-PDB). This narrow range is consistent with the low seasonality of oceanic low latitudes. The lower values are likely related to higher water availability and increased snail activity (values around -0.25‰ may correspond to rain water events while those more positive correspond to drier conditions). The d13C of body tissue ranges from -11.98‰ to -27.17‰ (V-PDB), suggesting that T. geminata specimens consumed both C3 and C4 plant types. The d13C values of snail shells range from -9.44‰ to 1.67‰. The values of d13C >1‰ exceed the natural range of values known for modern snails. The anomalously positive values likely reflect a diet rich in C4 plant and biogenic carbonates used in the synthesis of the shell (the values of d13C shell and body tissue are correlated, but some shell values are more positive than expected, suggestive of as much as 20 to 40% of biogenic sand carbonates in the diet). These results highlight the complex nature of the stable isotope signatures recorded by shells of land snails and also suggest that shells with more negative carbon isotope values would be the best choice for radiocarbon dating.