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

Abstract

We evaluated a 10 year time series of δ¹⁸O and δ¹³C records from three planktic foraminifers (<em>Neogloboquadrina pachyderma, Globigerina umbilicata, </em>and<em> Globigerinita glutinata</em>) in the Bering Sea and central subarctic Pacific with a focus on their responses to environmental changes. Foraminiferal δ¹⁸O followed the equilibriumequation for inorganic calcite, with species-specific equilibriumoffsets ranging fromnearly zero ( 0.02‰for <em>N. pachyderma </em>and 0.01‰ for <em>G.umbilicata</em>) to 0.16‰(<em>G. glutinata</em>). Equilibrium offsets in our sediment trap sampleswere smaller than those fromplankton tow studies, implying that foraminiferal δ¹⁸Owas modified by encrustation during settling. Habitat/calcification depths varied from 35-55m (<em>N. pachyderma</em> and <em>G.umbilicata</em>) or 25-45m (<em>G. glutinata</em>) duringwarm, stratified seasons to around 100m 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 <em>N. pachyderma</em> and <em>G. glutinata </em>are likely affected by other species-specific biological activities. Seasonal flux patterns reveal that fossil records of<em> N. pachyderma</em> and<em> G. glutinata </em>represent annual mean conditions, whereas that of<em> G.umbilicata</em> most likely indicates those of a specific season. Because none of these three taxawas abundant from December to February, their fossil records likely do not reflect isotope signals from cold seasons.