Seasonal Changes in the Isotopic Compositions and Sinking Fluxes of Euthecosomatous Pteropod Shells in the Sargasso Sea

Abstract

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 ofStyliola subula, Clio pyramidata, Limacina inflata, Creseis acicula, andCuvierina columnellawere collected throughout the sampling period. Comparisons of the δ18O of shell samples with the vertical and temporal variations in the calculated δ18O of aragonite in equilibrium with seawater suggest that these pteropods deposited the bulk of their shell mass at the following depths:S. subulaandL. inflataat 50 m,C. pyramidataat 75 m,C. aciculain the upper 25 m, andC. columnellaat 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 δ18O of shells were closely coupled to those of equilibrium δ18O for aragonite, suggesting that most of the shell mass of these individuals was formed within several months. Flux‐weighted, mean δ18O 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 exceptC. columnella, which suggests that temperature may influence the δ13C of the shells of these species.