The last glacial period was marked by pronounced millennial-scale variability in ocean circulation and global climate. Shifts in the sea ice cover of the Nordic Seas are believed to have amplified the glacial climate variability between warm interstadials and cold stadials in northern high latitudes and contributed to abrupt, high-amplitude temperature changes over Greenland. Here we present unprecedented empirical evidence that resolves the nature, timing, and role of sea ice fluctuations for abrupt ocean and climate change 32–40 thousand years ago, using biomarker sea ice reconstructions from the southern Norwegian Sea. Our results document that initial sea ice reductions at the core site preceded the major reinvigoration of convective deep-water formation in the Nordic Seas and heat release to the atmosphere during interstadials. Sea ice expansions initiated quickly after atmospheric peak warmth and preceded the buildup of a deep oceanic heat reservoir through stadials. Our findings suggest that the glacial variability in northern sea ice cover was essential in regime shifts between surface stratification and deep convection in the Nordic Seas, thus forming an important feedback for large-scale oceanic reorganization during abrupt glacial climate changes.

Supplement to: Sadatzki, Henrik; Dokken, Trond; Berben, Sarah M P; Muschitiello, Francesco; Stein, Ruediger; Fahl, Kirsten; Menviel, Laurie; Timmermann, Axel; Jansen, Eystein (2019): Sea ice variability in the southern Norwegian Sea during glacial Dansgaard-Oeschger climate cycles. Science Advances, 5(3), eaau6174