New bottom water reconstructions in the deep Norwegian Sea show higher [CO₃^2-] values in Marine Isotope Stages 5 and 4 than in the Holocene. This suggests modern-like/persistent deep-water formation in this region, even when Atlantic overturning weakened and/or shoaled. Our data put new constraints on the endmember [CO₃^2-] composition of northern component waters emerging from the Nordic Seas, with implications for the chemical characteristics and carbon storage capacity of the Atlantic Ocean.