Impact of abrupt sea ice loss on Greenland water isotopes during the last glacial period

Greenland ice cores provide excellent evidence of past abrupt climate changes. However, there is no universally accepted theory of how and why these Dansgaard-Oeschger (DO) events occur. Several mechanisms have been proposed to explain DO events, including sea ice, ice shelf buildup, ice sheets, atmospheric circulation, and meltwater changes. DO event temperature reconstructions depend on the stable water isotope (delta O-18) and nitrogen isotope measurements from Greenland ice cores: interpretation of these measurements holds the key to understanding the nature of DO events. Here, we demonstrate the primary importance of sea ice as a control on Greenland ice core delta O-18: 95% of the variability in delta O-18 in southern Greenland is explained by DO event sea ice changes. Our suite of DO events, simulated using a general circulation model, accurately captures the amplitude of delta O-18 enrichment during the abrupt DO event onsets. Simulated geographical variability is broadly consistent with available ice core evidence. We find an hitherto unknown sensitivity of the delta O-18 paleothermometer to the magnitude of DO event temperature increase: the change in delta O-18 per Kelvin temperature increase reduces with DO event amplitude. We show that this effect is controlled by precipitation seasonality.