Seasonal oxygen and carbon isotope variability in euthecosomatous pteropods from the Sargasso Sea

Abstract

We examine seasonal variations in the stable carbon and oxygen isotopic composition of individual shells of the pteropods Limacina inflata and Styliola subula, collected from Oceanic Flux Program sediment traps (at 500 m depth) near Bermuda in the western Sargasso Sea. Calcification depths estimated from L. inflata δ 18O vary between 200 and 650 m in late winter and spring, and between 50 and 250 m in late summer and fall. S. subula shows similar seasonal variability with calcification depths between 250 and 600 m in late winter and spring and 50–400 m in late summer and fall. These results suggest that both species calcify across a greater range of depths than indicated by previous geochemical studies. Furthermore, the data indicate that these species change their calcification depth in conjunction with changes in thermal stratification of the water column. Pteropod shell δ 13C values vary inversely with δ 13C DIC but show a positive correlation with seawater [CO 3 2−] and temperature after depth differences in δ 13C DIC are accounted for. We hypothesize that either the influence of temperature on metabolic CO 2 incorporation during shell growth and/or the influence of ambient [CO 3 2−] on shell geochemistry can explain these relationships. Taken together, the individual shell δ 18O and δ 13C data suggest that shell calcification, and by inference the life cycle, of these pteropods is several months or less. Individual pteropod shell analyses have potential for contributing to our understanding of the environmental parameters that play a role in seasonal calcification depth shifts, as well as to our knowledge of past upper ocean thermal structure.