Stable isotope and Sr/Ca profiles from the marine gastropod Conus ermineus: Testing a multiproxy approach for inferring paleotemperature and paleosalinity

摘要

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 O-18 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 inter-specimen Sr/Ca variability and error in the delta O-18(sw)-salinity regression. Profiles of delta C-13 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 C-13 variation reflects changes in the delta C-13 of dissolved inorganic carbon (delta C-13(DIC)). Salinity and delta C-13(DIC) at Stetson Bank strongly correlate (R-2 = 0.80, p < 0.0001), and shell delta C-13 minima coincide with local salinity minima following times of peak river discharge. These delta C-13 minima terminate during annual shelf current reversals. Low-salinity waters directly account for less than half the variability in shell delta C-13 but enhance summer stratification and trap respired CO2 from sediment pore waters. Specimens from this study show mean delta C-13 values 1 parts per thousand lower than C ermineus collected from Stetson Bank in 1971, reflecting the decrease in delta C-13(DIC) from anthropogenic CO2.