Modal shift in North Atlantic seasonality during the last deglaciation

Abstract

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.

Taxonomic review of living planktonic foraminifera

“…At present we refrain from using such characters, but note that in some instances, these differences could be resolved in the fossil record by elemental and isotopic signatures in the shell (e.g. Brummer et al, 2020). For a comprehensive discussion on a possible method to establish for genetically circumscribed taxa a stable nomenclature that exists outside of the Code, the reader is referred to the work by Morard et al (2016).…”

Last Deglacial Environmental Change in the Tropical South Pacific From Tahiti Corals

“…According to the Milankovitch hypothesis, the transition from glacial to interglacial climate states is initiated by an increase in the Northern Hemisphere summer insolation, while total annual insolation remains nearly constant (Berger, 1980). Although insolation seasonality is a fundamental driver of past climate variability, there is only little paleoclimatological proxy data with sufficient resolution to resolve climate variations on the sub‐annual scale and reconstruct the amplitude of annual cycles of the past (Brummer et al., 2020). For instance, sediment cores covering the last deglacial transition in the central Pacific are sparse and exhibit very low sedimentation rates of ∼1 cm ka −1 (Seo et al., 2016).…”

Shell Reworking Impacts on Climate Variability Reconstructions Using Individual Foraminiferal Analyses

“…Over the past 15 years, the analysis of stable oxygen isotope (δ 18 O) and magnesium to calcium (Mg/Ca) ratios in individual shells of planktic foraminifera—termed individual foraminiferal analyses (IFA, Thirumalai et al., 2013)—has gained momentum as a powerful tool to reconstruct seasonal and/or interannual oceanic (and climatic) variability in various oceanographic settings. IFA data have laid the foundations for a deeper understanding of high‐frequency climate variability in the past, for example, the El Niño–Southern Oscillation (ENSO) system (Ford et al., 2015; Khider et al., 2011; Koutavas & Joanides, 2012a; Koutavas et al., 2006; Leduc et al., 2009; Rustic et al., 2020; Scroxton et al., 2011; White et al., 2018), seasonality in the North Atlantic Ocean and Arabian Sea (Brummer et al., 2020; Metcalfe et al., 2019; Naidu et al., 2019), the recently hypothesized equatorial Indian Ocean mode of variability during the Last Glacial Maximum (LGM, Thirumalai et al., 2019b). One emerging paradigm from IFA studies points to the mean state of global (and regional) climate as a modulator of the superimposed, shorter‐term climate variability (Ford et al., 2015; Rustic et al., 2020; Thirumalai et al., 2019b).…”