Stable isotope (delta O-18, delta C-13 and delta D) signatures of recent terrestrial communities from a low-latitude, oceanic setting: Endemic land snails, plants, rain, and carbonate sediments from the eastern Canary Islands

摘要

Stable isotopes of oxygen and carbon extracted from fossil land snail shells have been used increasingly to interpret past environments. To evaluate the utility of this approach for low-latitude oceanic islands, populations of the modem helicid land snail Theba geminata - a species also abundant in the Quaternary fossil record of the region - were sampled at ten low altitude (< 300 m) sites from coastal areas of the eastern Canary Islands. The results include stable isotopes of (1) 17 aragonite shells of live-collected adult snails-, (2) 17 body tissue samples from the same snail individuals; (3) 10 samples of carbonate sediments; (4) 69 plant tissue samples representing all 24 identified species; and (5) 7 rain water samples. The mean isotopic composition of the rain water is - 5 parts per thousand (V-SMOW) for 61) and -2 parts per thousand (V-SMOW) for 6180, ranging from - 11 parts per thousand to +2 parts per thousand (V-SMOW) and from - 2.6 parts per thousand to -0.7 parts per thousand (V-SMOW), respectively. The local vegetation is heterogeneous, including C-3, C-4, and CAM plants. delta C-13 values vary from - 13.0 parts per thousand to -29.0 parts per thousand (V-PDB) across plant species. Of the 24 species, five are C-4, 15 are C-3, and four are CAM plants. The delta O-18 values for shells represent a narrow range of values (from -0.3 parts per thousand to +2.5 parts per thousand [V-PDB]), which is consistent with the low climate seasonality typifying low-latitude oceanic settings. Hypothetical model of the expected delta O-18 value for shell aragonite precipitated in equilibrium suggests that the most negative delta O-18(shell) represent the closest estimate for delta O-18(rain water). The delta C-13 values of shells range from - 9.4 parts per thousand to + 1.7 parts per thousand (V-PDB). The Most Positive delta C-13 values are attributed to a diet based on C4 plants. The comparison of delta C-13 values of soft tissues and shells suggests that snails ingested notable amounts (from similar to 20% up to similar to 40%) of foreign carbonates. Consequently, fossil shells with the most negative 613C values should be selected for radiocarbon dating in future geochronological studies of the region. The delta C-13 values of body tissues vary from -12.0 parts per thousand to -27.2 parts per thousand (V-PDB), indicating that land snails consumed C-3 and C-4 plants indiscriminately. The mean carbon isotopic composition, averaged across multiple fossil specimens, may thus provide a useful tool for reconstructing paleoclimates and paleoenvironments throughout the Quaternary history of the Canary Islands and other comparable low-latitude oceanic settings.