D. Keith Gentry, Sindia Sosdian, Ethan L. Grossman, Yair Rosenthal, David Hicks, and Caroline H. Lear
Stable isotope and Sr/Ca profiles from the marine gastropod Conus ermineus; testing a multiproxy approach for inferring paleotemperature and paleosalinity
Palaios (April 2008), 23(4):195-209

Abstract:
This study tests the fidelity of shallow-water gastropod skeletons as multiproxy archives of paleoenvironmental change by comparing isotopic and trace-metal analyses of specimens of Conus ermineus. Four adult specimens were collected live from Stetson Bank in the northwestern Gulf of Mexico during the summer of 2003. Shells were sampled along axes of growth to produce time-series profiles spanning up to 8 years. delta ¹⁸ O and Sr/Ca profiles show seasonal cyclicity modified by fast summer and slow winter shell growth. The profiles were combined to estimate paleosalinity. This yields variable results that overestimate salinity range; nevertheless, annual salinity minima and maxima are still evident. The overestimates are attributed to interspecimen Sr/Ca variability and error in the delta ¹⁸ O _sw -salinity regression. Profiles of delta ¹³ C show seasonal variation superimposed on a decreasing ontogenetic trend, the latter ascribed to decreasing metabolic efficiency also reflected by an ontogenetic increase in Sr/Ca. Seasonal delta ¹³ C variation reflects changes in the delta ¹³ C of dissolved inorganic carbon (delta ¹³ C _DIC ). Salinity and delta ¹³ C _DIC at Stetson Bank strongly correlate (R ² = 0.80, p<0.0001), and shell delta ¹³ C minima coincide with local salinity minima following times of peak river discharge. These delta ¹³ C minima terminate during annual shelf current reversals. Low-salinity waters directly account for less than half the variability in shell delta ¹³ C but enhance summer stratification and trap respired CO ₂ from sediment pore waters. Specimens from this study show mean delta ¹³ C values 1 per mil lower than C. ermineus collected from Stetson Bank in 1971, reflecting the decrease in delta ¹³ C _DIC from anthropogenic CO ₂ .