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.
Highlights
1.1 Seasonality and single-foraminiferal analysis (SFA)Stable oxygen isotopes (δ18O) of pooled foraminifera have been used as key tracers of water masses (e.g. Epstein and Mayeda, 1953; Frew et al, 2000), ice-volume and sea-level fluctuations (e.g. Grant et al, 2012; Waelbroeck et al, 2002; Shackleton, 1987) over glacial–interglacial cycles (e.g. Pearson, 2012; Waelbroeck et al, 2005)
Our glacial results show the abundance during this period has two peaks centred at 515 and 560 cm, with a large proportion of the data occurring within an interval of lower abundance
Our results show that δ18O values of both G. bulloides and N. pachyderma are predominately unimodally distributed during the last glacial until about 21 ka (Figs. 5 and 6)
Summary
1.1 Seasonality and single-foraminiferal analysis (SFA)Stable oxygen isotopes (δ18O) of pooled foraminifera have been used as key tracers of water masses (e.g. Epstein and Mayeda, 1953; Frew et al, 2000), ice-volume and sea-level fluctuations (e.g. Grant et al, 2012; Waelbroeck et al, 2002; Shackleton, 1987) over glacial–interglacial cycles (e.g. Pearson, 2012; Waelbroeck et al, 2005). Leduc et al, 2009; Koutavas et al, 2006; Koutavas and Joanides, 2012; Scussolini et al, 2013) that includes seasonal differences (Feldmeijer et al, 2015; Ganssen et al, 2011; Metcalfe et al, 2015; Wit et al, 2010) Seasonal changes during these glacial–interglacial cycles have rarely been addressed, resolving seasonal contrasts would significantly improve our understanding of past climate change (Huybers, 2006; Schmittner et al, 2011)
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