Deglaciation Changes the North Atlantic Seasonality

Anthropogenic warming is currently causing a pole-ward retreat of the global sea ice distribution (Stocker et al., 2013). Reduced summer sea ice allows for solar irradiation and wind mixing that promote ocean productivity and atmospheric CO₂ drawdown at elevated temperatures. Here we show that the past natural deglaciation was accompanied by punctuated shifts in the seasonal succession and species productivity of planktonic foraminifera now found in the modern polar, subpolar and temperate North Atlantic, respectively. We identify a similar succession in a single sediment core during the last glacial-interglacial transition using single shell oxygen isotopes of “polar” Neogloboquadrina pachyderma (left coiling) and “subpolar” Globigerina bulloides. Glacial productivity is limited to a single maximum in late summer and dominated by N. pachyderma, followed by melting icebergs and winter sea ice. Deglaciation shifted the main plankton bloom towards early summer, adding a second “warm” population of N. pachyderma with G. bulloides in between. At the end of the last deglaciation first “cold” then “warm” N. pachyderma become extinct by exceeding threshold temperatures while G. bulloides persists at the core location. We conclude that polar shifts in seasonal timing and productivity structure, resuming in response to the current Anthropogenic warming, are resolved by single shell δ¹⁸O of planktonic foraminifera.