干旱区生态环境知识资源中心

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. (C) 2008 Elsevier B.V All rights reserved.