STABLE ISOTOPIC RECORD OF SEASONAL DROUGHT IN LAKE WATER AND MOLLUSK CARBONATE; WOODMAN POND, CENTRAL NEW YORK STATE

An eight-year record of stable isotopes (H, C, O) in lake water and gastropod carbonate in a hardwater lake in central New York State demonstrates that summer drought conditions are recorded as positive oxygen isotope signatures in Viviparus georgianus. Water samples collected from the inlet and outlet of Woodman Pond, a 92-acre marl lake show that evaporative effects during prolonged summer drought intervals raise δD and δ¹⁸O of outflowing lake water relative to more negative inflowing water. Inflow water has an isotope signature dominated by groundwater contributions influenced by winter snow melt with relatively more negative isotope signatures (δD = -75 to -66‰; δ¹⁸O_SMOW= -10.6 to -9.3‰. Outlet waters (δD = -69 to -59‰; δ¹⁸O_SMOW= -9.2 to -7.8‰ define a clear evaporative isotopic trend relative to the Meteoric Water Line.

The gastropod V. georgianus lives in shallow water along the lake margin and forms strongly-defined annual growth increments during the summer months. Oxygen stable isotopes (overall range of δ¹⁸O_PDB is -10.3 to -7.2‰) of shell carbonate show year-to-year variation that broadly follows stable isotopic signatures of lake outlet water, and are correlative to the summer month Palmer Drought Severity index for the central New York region over the years of record. Covariance of carbon (overall range of δ¹³C is -13.7 to -8.0‰) and oxygen stable isotopes in shell carbonate suggests that removal of organic carbon in upper lake waters is enhanced during relatively dry summer periods, a pattern recognized in other marl lakes. Our data demonstrate that local temperature and rainfall conditions of relatively short duration can be recorded as significant stable isotopic excursions in gastropod shell carbonate. The utility of lacustrine mollusks as paleohydrologic proxies may be limited by the ease with which short-term evaporative effects are recorded, as observed in Woodman Pond.