Seasonal Changes in the Isotopic Compositions and Sinking Fluxes of Euthecosomatous Pteropod Shells in the Sargasso Sea
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Seasonal variations in the oxygen and carbon isotopic compositions and fluxes of five euthecosomatous pteropods were determined from a 14-month series of sediment trap deployments in the Sargasso Sea. Medium and large shell sizes of Styliola subula, Clio pyramidata, Limacina inflata, Creseis acicula, and Cuvierina columnella were collected throughout the sampling period. Comparisons of the delta O-18 of shell samples with the vertical and temporal variations in the calculated delta O-18 of aragonite in equilibrium with seawater suggest that these pteropods deposited the bulk of their shell mass at the following depths: S. subula and L. inflata at 50 m, C. pyramidata at 75 m, C. acicula in the upper 25 m, and C. columnella at 50-75 m. Although several of these species undergo diel vertical migration of several hundred meters in this region, the estimated depths of calcification match the upper parts of the species’ vertical ranges, where the mean populations occur only at night. in all species, seasonal changes in the delta O-18 of shells were closely coupled to those of equilibrium delta O-18 for aragonite, suggesting that most of the shell mass of these individuals was formed within several months. Flux-weighted, mean delta O-18 values for the species reveal that seasonal variations in the sinking fluxes of shells would not affect the isotopic compositions of shell accumulations in Bermuda Rise sediments. Carbon and oxygen isotopes were positively correlated in all species except C. columnella, which suggests that temperature may influence the delta O-13 of the shells of these species.