Oxygen Isotope Variability of Planktonic Foraminifera Provide Clues to Past Upper Ocean Seasonal Variability

The major control upon abundance of planktonic foraminifera and their stable oxygen isotope (delta O-18) signature is the seasonally linked variation in water hydrography, key to the proliferation or attenuation of ecologically beneficial constraints. The range and variance sigma(delta O-18) of planktonic foraminifera can reflect changes in either the season or depth of calcification. For a detailed reconstruction of ocean changes we employed multispecies single-specimen analysis, which allows extraction of the isotopic variability within the species for the time covered by the sample. Previous studies with pooled specimens have shown that the multiannual temperature range may be extracted. Here we investigate how seasonality can be deduced from single-specimen analysis of planktonic foraminifera combined with multiple other proxies (IRD percent, faunal abundance) from Termination III. Our single-shell isotope results show that the variance in Globigerina bulloides oxygen isotope values corresponds to the insolation at the core site. Furthermore, faunal and isotopic analyses of the polar-subpolar neogloboquadrinid species, N. pachyderma (NPS) and N incompta, reveal an intriguing result. These species are sister taxa, representing genetically distinct species, whose relative abundance reflects warm and cold conditions. While the difference between their isotopic means should reflect the temperature difference between their distinct growing seasons, we show that this difference also has a statistically significant relationship with the spread in individual NPS delta O-18. At an appropriate core site, this approach could be used to further constrain the length of the growing season and therefore the inherent variability recorded within proxy records. Plain Language Summary Reconstructions of the past climate of the Earth have focused upon the average signal, either mean annual temperature or distinct seasons (e.g., average summer or winter), as recorded by the shells of planktonic organisms in deep-sea sediments. However, these records are biased by the waxing and waning of environmental variables that benefit or hinder the growth and proliferation of individual species. The spread in isotope values recorded by individual shells can therefore give more information as to the extent of the growing period as well as provide information upon the seasonality of the past.