Geochemical and morphological characteristics of Globorotalia truncatulinoides, a deep dwelling planktonic foraminifer, have been used since the mid-1950s to infer (paleo)oceanographic conditions of the upper ocean. The coiling ratio has been linked to different water masses and stable oxygen isotope signal of this species to changes in depth habitat and/or season. Here we show that the isotopic composition of single specimens covering Termination III of multiple size fractions of North Atlantic G. truncatulinoides sinistral is indicative of a deeper calcification depth in the water column compared to G. truncatulinoides dextral as previously indirectly inferred in a plankton tow study. Furthermore, the change in coiling ratio from dominantly G. truncatulinoides dextral (95%) to brief episodes of dominantly G. truncatulinoides sinistral (80%) gives a strong indication of deepening of the permanent thermocline during periods in which G. truncatulinoides sinistral was dominant. The position of the permanent thermocline during marine isotope stages 8 and 7 echoes the relative strength of the Atlantic meridional overturning circulation (AMOC), dominated by interglacial-glacial dynamics. We demonstrate that Glacial Heinrich (ice-rafted debris) events appear to proceed a permanent thermocline shoaling, whereas interglacial Heinrich events follow the shoaling of the permanent thermocline, likely a result of a weakened AMOC.

Abstract. So-called "vital effects" are a collective term for a suite of physiologically and metabolically induced variability in oxygen (δ18O) and carbon (δ13C) isotope ratios of planktonic foraminifer shells that hamper precise quantitative reconstruction of past ocean parameters. Correction for potential isotopic offsets from equilibrium or the expected value is paramount, as too is the ability to define a comparable life stage for each species that allows for direct comparison. Past research has focused upon finding a specific size range for individual species in lieu of other identifiable features, thus allowing ocean parameters from a particular constant (i.e. a specific depth or season) to be reconstructed. Single-shell isotope analysis of fossil shells from a mid-latitude North Atlantic Ocean piston core covering Termination III (200 to 250 ka) highlight the advantage of using a dynamic size range, i.e. utilising measurements from multiple narrow sieve size fractions spanning a large range of total body sizes, in studies of palaeoclimate. Using this methodology, we show that isotopic offsets between specimens in successive size fractions of Globorotalia inflata and Globorotalia truncatulinoides are not constant over time, contrary to previous findings. For δ18O in smaller-sized globorotalids (212–250 μm) it is suggested that the offset from other size fractions may reflect a shallower habitat in an early ontogenetic stage. A reduction in the difference between small and large specimens of G. inflata between insolation minima and maxima is interpreted to relate to a prolonged period of reduced water column stratification. For the shallow-dwelling species Globigerina bulloides, no size–isotope difference between size fractions is observed, and the variability in the oxygen isotopic values is shown to correlate well with the seasonal insolation patterns. As such, patterns in oxygen isotope variability of fossil populations may be used to reconstruct past seasonality changes.

[1] Paleoceanographic studies routinely combine different foraminiferal proxies (i.e., weight, abundance, trace metal, and stable isotope measurements) into a cohesive narrative. The application of chemical treatment to disaggregate ocean sediments in the most efficient way to isolate the fossils of foraminifera from the other sediment components is dictated by the time available and the material used. Yet few studies have aimed to test both the physical and geochemical effects associated with such practices. In this study, we use samples with different sedimentological characteristics (i.e., varying percentages of CaCO 3 and of terrigenous material) to test the impact upon these proxies of three processing methods and a control: (1) no chemicals (contol run); (2) soaking in sodium hexametaphosphate (CalgonV R ); (3) soaking in hydrogen peroxide; and (4) soaking in a sodium pyrophosphate. The samples were analyzed for faunal abundance, shell weight, stable isotope ( 18 O, 13 C), and trace metal (Mg/Ca) geochemistry for four species of planktonic foraminifera (Globigerina bulloides, Globigerinoides ruber, Globorotalia inflata, and Globorotalia menardii). Results show that apart from the CalgonV R solution, the values of faunal abundance, shell weight, Mg/Ca, and stable isotopes are similar irrespective of the cleaning treatment utilised and therefore warrant cross-comparison of results obtained with different preparation techniques. The use of CalgonV R in pretreatment shows statistically different values for only foraminiferal shell weight.

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.

Abstract. Changeover from a glacial to an interglacial climate is considered as transitional between two stable modes. Palaeoceanographic reconstructions using the polar foraminifera Neogloboquadrina pachyderma highlight the retreat of the Polar Front during the last deglaciation in terms of both its decreasing abundance and stable oxygen isotope values (δ18O) in sediment cores. While conventional isotope analysis of pooled N. pachyderma and G. bulloides shells shows a warming trend concurrent with the retreating ice, new single-shell measurements reveal that this trend is composed of two isotopically different populations that are morphologically indistinguishable. Using modern time series as analogues for interpreting downcore data, glacial productivity in the mid-North Atlantic appears limited to a single maximum in late summer, followed by the melting of drifting icebergs and winter sea ice. Despite collapsing ice sheets and global warming during the deglaciation, a second “warm” population of N. pachyderma appears in a bimodal seasonal succession, separated by the subpolar G. bulloides. This represents a shift in the timing of the main plankton bloom from late to early summer in a “deglacial” intermediate mode that persisted from the glacial maximum until the start of the Holocene. When seawater temperatures exceeded the threshold values, first the “cold” (glacial) then the “warm” (deglacial) populations of N. pachyderma disappeared, whilst G. bulloides with a greater tolerance to higher temperatures persisted throughout the Holocene to the present day in the midlatitude North Atlantic. Single-specimen δ18O of polar N. pachyderma reveals a steeper rate of ocean warming during the last deglaciation than appears from conventional pooled δ18O average values.