Seasonal variability of δ¹⁸O and δ¹³C of planktic foraminifera in the Bering Sea and central subarctic Pacific during 1990-2000

We evaluated a 10 year time series of δ¹⁸O and δ¹³C records from three planktic foraminifers (Neogloboquadrina pachyderma, Globigerina umbilicata, and Globigerinita glutinata) in the Bering Sea and central subarctic Pacific with a focus on their responses to environmental changes. Foraminiferal δ¹⁸O followed the equilibrium equation for inorganic calcite, with species-specific equilibrium offsets ranging from nearly zero (-0.02‰ for N. pachyderma and -0.01‰ for G. umbilicata) to -0.16‰ (G. glutinata). Equilibrium offsets in our sediment trap samples were smaller than those from plankton tow studies, implying that foraminiferal δ¹⁸O was modified by encrustation during settling. Habitat/calcification depths varied from 35-55 m (N. pachyderma and G. umbilicata) or 25-45 m (G. glutinata) during warm, stratified seasons to around 100 m during winter, when the mixed layer depth increases. Unlike δ¹⁸O, foraminiferal δ¹³C showed species-specific responses to environmental changes. We found a dependency of δ¹³C in G. umbilicata on CO₃^2- concentrations in ambient seawater that agreed reasonably well with published laboratory results, suggesting that δ¹³C of G. umbilicata is subject to vital effects. In contrast, δ¹³C of N. pachyderma and G. glutinata are likely affected by other species-specific biological activities. Seasonal flux patterns reveal that fossil records of N. pachyderma and G. glutinata represent annual mean conditions, whereas that of G. umbilicata most likely indicates those of a specific season. Because none of these three taxa was abundant from December to February, their fossil records likely do not reflect isotope signals from cold seasons.