Persistent shallow subsurface warming during North Atlantic Stadials drives ice-destabilisation and rapid climate change

The last ice age was repeatedly punctuated by millennial scale intervals of extreme cold in the Northern Hemisphere. These cold periods, known as stadials, are also times of ice calving and glacial melt, sending debris-laden icebergs into the North Atlantic Ocean, and raising questions about the mechanisms of ice sheet instability. Here, we provide new high resolution marine temperature reconstructions from the northeast north Atlantic (ODP 980 55°29.1’N 14°42.1’W) that show that, whilst persistent cold stadial temperatures might be inferred from abundances of the polar foraminifera Neogloboquadrina pachyderma (% N. pachyderma), Mg/Ca temperature reconstructions from the same planktic species and Globigerina bulloides indicate gradual subsurface warming by as much as 3ᵒC over the course of a stadial. We explain this apparent discrepancy by turning to the seasonal influences on these temperature proxies, suggesting that very high % N. pachyderma reflects preferential survival of this species in polar waters with extensive sea ice, while Mg/Ca reveals relatively mild subsurface conditions during the summer growing season. Modelling shows that warming summers in the high latitude subsurface may be explained by persistent influence of warm waters from lower latitudes, in combination with a lack of winter heat loss due to the insulating effect of sea ice and climbing atmospheric CO₂. This accumulation of heat at critical depths for marine terminating glaciers underlines the influence of warming seawater on ice sheet stability and could provide an additional source of heat to trigger abrupt interstadial warming.