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

Abstract

The major control upon abundance of planktonic foraminifera and their stable oxygen isotope (δ18O) signature is the seasonally linked variation in water hydrography, key to the proliferation or attenuation of ecologically beneficial constraints. The range and variance σ(δ18O) 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 δ18O. 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.

Modal shift in North Atlantic seasonality during the last deglaciation

“…Pearson, 2012;Waelbroeck et al, 2005). Technical advances allow (Killingley et al, 1981;Schiffelbein and Hills, 1984;Oba, 1990Oba, , 1991Billups and Spero, 1996) for the routine analysis of the stable isotopic composition of single microscopic shells Lougheed et al, 2018;Metcalfe et al, 2015Metcalfe et al, , 2019Frizt-Endres et al, 2019;van 266 G.-J. A. Brummer et al: Modal shift in North Atlantic seasonality during the last deglaciation water temperature (Ganssen et al, 2011;Wit et al, 2010;Groenveld et al, 2019), in lieu of pooled specimens that capture an averaged state of the system on longer timescales.…”

“…It is important to note that G. bulloides also on occasion has more than a single population at this core site; however, the species' cosmopolitan and optimistic nature make it less surprising that expansion of seasonal variables that intersects the species' tolerances will lead to an expansion of its ecological range (e.g. Metcalfe et al, 2019). The appearance of a second population in N. pachyderma is more surprising because ecological expansion for this species can only be unidirectional (i.e.…”

SEAMUS (v1.20): a Δ<sup>14</sup>C-enabled, single-specimen sediment accumulation simulator

“…Moreover, machine analysis of multi-specimen samples will only report the mean value and machine error, thus hiding the true distribution of values within the sample. Advances in mass spectrometry eventually allowed the analysis of single specimens (Killingley et al, 1981) and, since single specimens capture a single year or season of the climate signal, researchers can in principle study the full distribution of isotope or trace element values obtained within various discrete depths of sediment cores, thereby making inferences regarding variability in climate, habitat or specimen morphology for various specific time periods during the Earth's history (Spero and Williams, 1990;Tang and Stott, 1993;Billups and Spero, 1996;Ganssen et al, 2011;Wit et al, 2013;Ford et al, 2015;Metcalfe et al, 2015;Ford and Ravelo, 2019;Metcalfe et al, 2019b). However, the accuracy with which the aforementioned studies can quantify time-specific variation for a particular climate period, habitat or morphological variable is strongly dependent upon the constraint of the age range of the specimens contained within a given discretedepth interval.…”

SEAMUS (v1.0): a Δ<sup>14</sup>C-enabled, single-specimen sediment accumulation simulator

“…Moreover, machine analysis of multi-specimen samples will only report the mean value and machine error, thus hiding the true distribution of values within the sample. Advances in mass spectrometry eventually allowed the analysis of single specimens (Killingley et al, 1981) and, since single specimens capture a single year/season of the climate signal, researchers can study the full distribution of isotope or trace element values obtained from single specimens contained within various discrete depths of sediment cores to make inferences regarding variability in climate, habitat or specimen morphology for various specific time periods during the Earth's history (Spero and Williams, 1990;Tang and Stott, 1993;Billups and Spero, 1996;Ganssen et al, 2011;Wit et al, 2013;Ford et al, 2015;Metcalfe et al, 2015Metcalfe et al, , 2019bFord and Ravelo, 2019). However, the accuracy with which the aforementioned studies can quantify time-specific variation for a particular climate period, habitat or morphological variable is strongly dependent upon the constraint of the age range of the specimens contained within a given discretedepth interval.…”