Modal shift in North Atlantic seasonality during the last deglaciation.

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. [ABSTRACT FROM AUTHOR]