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

Abstract

AbstractWe 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 CO32− 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.