Today, the sub-surface Denmark Strait Overflow (DSO) and the Iceland-Scotland Overflow form the starting points of the Atlantic Meridional Overturning Circulation and compensate for the poleward flowing Norwegian and Irminger branches of the North Atlantic surface current that drive the 'Nordic Heat Pump'. During peak glacial and early deglacial times, ice sheets on Iceland and Greenland, and ice-induced isostatic and eustatic sea-level changes reduced the Denmark Strait aperture and DSO. Yet, extremely high benthic stable carbon and oxygen isotope values together with very high ventilation ages of bottom waters suggest a north-south density gradient of intermediate-waters and persistent flow of partially Arctic-sourced waters through both Denmark Strait and Faeroe Channel, analogous to today. The arrival of deglacial meltwaters off northern Iceland induced the onset of Heinrich-Stadial 1 near 18.400 yr BP, as derived from 14C-plateau tuning. They caused a tipping point in DSO circulation shown by 3 °C warming, reduced ventilation and ventilation ages of bottom water, moreover, by increased radiogenic Nd isotope signatures at luff-side Site PS2644. These records suggest a sudden subsurface incursion of Atlantic intermediate waters across basaltic sediments from S.E. of Iceland. Deep-water convection off Norway then was replaced by weak brine water formation, coeval with a breakdown of the 'Nordic Heat Pump' evidenced by a temperature drop on Greenland. After 16.2 cal ka, a major meltwater outbreak from the Barents ice shelf led to modified Heinrich-1-style circulation until ∼15.1 cal. ka. Conversely, the DSO intensified during interstadial and Holocene times, causing sediment hiatuses at Site PS2644.