STABLE ISOTOPIC COMPOSITION OF THE FRESHWATER GASTROPOD VIVIPARUS GEORGIANUS, CENTRAL NEW YORK STATE: EVALUATION AS A PROXY FOR SUMMER CLIMATE

Viviparous georgianus, a common gastropod species found in temperate climate marl lakes in eastern North America, records changes in the oxygen isotopic composition of surface water in Woodman Pond, a 92-acre hardwater kettle lake in central New York State. Annual differences in shell isotopic composition are controlled by the isotopic composition of shallow lake water, which varies due to changes in the proportion of surface and groundwater infl ow versus evaporation. An eight-year record of water isotopic composition, gastropod shell isotopic composition and climate data suggest that δ 18 O of gastropod shell correlates with average summer month values of the Palmer Drought Severity Index (PDSI), a measure that combines precipitation and evaporation potential. The most positive oxygen isotope values of gastropod shell carbonate are correlated with the most negative (hot, dry) summer values of the PDSI. Surface waters sampled at inlet and outlet sites in Woodman Pond during summers of 1999-2004 have δ 18 O SMOW ranging from -10.9 to -8.3‰. The most positive values are from outlet samples taken during relatively dry, hot summer periods. Outlet samples lie to the right of the Meteoric Water Line on a bivariate δ 18 O vs δ 2 H plot, whereas inlet samples lie to the left, indicating that evaporation of surface lake water is an important control on water isotopic composition during summer. Oxygen stable isotopic composition of gastropod calcite varies from -9.8 to -7.3 ‰ PDB, consistent with equilibrium precipitation of calcite at lake water temperatures of 20-28 o C. A modest co-variance of δ 18 O and δ 13 C in the shell carbonate is observed, and is related to depletion of organic carbon from lake surface waters during the warmest mid-summer periods when evaporation is also greatest.