A seasonally ice-free Arctic Ocean during the Last Interglacial

The extent of Arctic sea-ice during the Last Interglacial is poorly known. Climate models and sediment-based reconstructions generally suggest a relatively extensive ice cover, comparable to the modern day. Here, we show that Arctic sea-ice was much more reduced than previously assumed, with summers being ice-free. Our new evidence stems from a series of central Arctic Ocean sediment cores, including sites that underlie the thickest parts of the modern Arctic ice pack. Microfossil analysis reveals that the Arctic Ocean was invaded by Turborotalita quinqueloba, a typically subpolar planktonic foraminifer that is strongly associated with chilled Atlantic waters in the modern North Atlantic Ocean, and which is absent in modern sediments in the central Arctic Ocean. Given that the modern Arctic Ocean is characterised by a pronounced halocline with Atlantic waters subducting beneath a fresh and cool upper water mass, our findings suggest a shallowing of those Atlantic waters in the Arctic Ocean during the Last Interglacial. This process, dubbed ‘atlantification’, would be associated with retreating sea-ice, allowing T. quinqueloba to invade. Since the onset of the atlantification of the Arctic Ocean in response to climate change is increasingly being reported, we suggest that the Last Interglacial may serve as an important analogue for studying a fully-atlantified, seasonally ice-free Arctic Ocean.