Quaternary interglacial environmental stability in San Salvador Island (Bahamas): A land snail isotopic approach

The C-13/C-12 and O-18/O-16 ratios of modern and fossil shells of two ecologically-distinct land snail taxa (Cerionidae and Annularidae) from San Salvador Island were analyzed to estimate the interglacial Pleistocene and Holocene paleoenvironmental conditions in the Bahamas. The average delta C-13 value of Cerion was -5.0 +/- 1.8 parts per thousand for modern (similar to 0-0.1 ka) specimens, -4.3 +/- 0.7 parts per thousand for late Holocene (similar to 2-4.6 ka) individuals, -3.4 +/- 1.6 parts per thousand for middle Holocene (similar to 5-63 ka) shells, -4.0 +/- 1.5 parts per thousand for similar to 85 or similar to 110 ka specimens, and -3.6 +/- 0.4 parts per thousand for individuals that grew during OIS 5e (similar to 125 ka). Annularidae specimens displayed average delta C-13 values of -6.3 +/- 2.5 parts per thousand, -6.1 +/- 1.0 parts per thousand, -2.4 +/- 1.0 parts per thousand, -4.4 +/- 1.1 parts per thousand, and -5.0 +/- 2.9 parts per thousand, respectively. These results suggest that snails consumed C-3 and C-4 plants arbitrarily through time, however, higher proportion of C-4 plants may have been consumed in the past compared to the present. Cerion was similar to 1 parts per thousand higher in average delta C-13 value than Annularidae specimens for some time-intervals, indicating greater C-4 plant consumption and/or higher ingestion of carbonate-rich sediments. The average delta O-18 value of Cerion was -05 +/- 0.8 parts per thousand for modern individuals, -05 +/- 0.5 parts per thousand for late Holocene specimens, -0.8 +/- 0.2 parts per thousand for middle Holocene shells, -0.4 +/- 0.6 parts per thousand for similar to 85 or similar to 110 ka specimens, and -0.3 +/- 0.6 parts per thousand for OIS 5e individuals. Annularidae individuals exhibited average delta O-18 values of -0.7 +/- 0.8 parts per thousand, -0.5 +/- 0.9 parts per thousand, -0.9 +/- 0.3 parts per thousand, -0.3 +/- 0.6 parts per thousand and -0.9 +/- 0.9 parts per thousand, respectively. These results point to similar atmospheric conditions over time-intervals when shell was deposited. The shell delta O-18 values did not significantly differ between species despite the apparent differences in lifestyle. Calculations from a snail evaporative steady-state flux balance model and proxy data suggest that modem individuals deposited shell at air temperatures of similar to 24-25 degrees C, ambient water delta O-18 values between -4.5 parts per thousand and -3.5 parts per thousand (SMOW), and relative humidity of similar to 87-89%. Late to middle Holocene and interglacial late Pleistocene shells appear to have precipitated under a similar environmental setting. Overall, atmospheric conditions during these Quaternary interglacial time-intervals at coastal sites of San Salvador were largely comparable to the present. This apparent environmental similarity is consistent with the relatively comparable snail richness and diversity observed at those same locales and time-intervals. (C) 2012 Elsevier B.V. All rights reserved.