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 (δ ¹⁸ O) signature is the seasonally linked variation in water hydrography, key to the proliferation or attenuation of ecologically beneficial constraints. The range and variance σ(δ ¹⁸ O) 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 δ ¹⁸ O. 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.