Deep north atlantic circulation strength: Glacial-interglacial variability over the last 400,000 years

Most of the high-resolution studies investigating the past variability of the deep North Atlantic circulation are focused on recent timescales, but little is known for periods older than the last glacial period. Based on magnetic, elemental and physical grain-size proxies of three pairs of sediment cores located South of Iceland, we reconstructed the past changes in grain-size and basaltic-derived grain concentration over the last 400 ka (1 ka = 1000 years). The source and grain-size proxies exhibit highly similar variations between the three sites, suggesting they are affected by the same regional process. Persistently lower concentrations and finer grain-sizes recorded in the southernmost and deeper site compared to the northern sites indicate a North to South gradient that is consistent with the southward transport of sediments from a northern basaltic source by a deep current. Therefore, we interpret changes recorded by the ensemble of proxies at the three sites as past variations in the ISOW intensity. These new results show persistent influence, over the entire 400 ka, of a southward deep-water flow in the subpolar North Atlantic, from 1800 to 2800 m water depth. The “off mode” of deep Atlantic circulation during Heinrich events suggested by many studies is therefore questioned. This study extends the previous observations made for the 20–65 ka period over the last 400 ka. Our results also show that the three studied sites are bathed by a deep-water mass formed in the Nordic Seas, during both glacial and interglacial periods, suggesting that the present-day convection areas were still active during glacial periods. Our ISOW intensity records are highly similar to those of deep-water ventilation. We propose that the strength of overflows in the North Atlantic regulates the volume of North Atlantic Deep Water (NADW) and its southward transport in the Atlantic basin. Therefore, the Southern source waters may only act as an opportunistic component of the deep Atlantic circulation, invading or retreating depending on the NADW strength. • All proxies used in this study consistently record the same variability of the past strength of Iceland Scotland Overflow Water (ISOW). • Our results indicate a deep-current continuously flowing southward over the last 400 ka, suggesting no “off-mode” of AMOC in this area, always bathed by the (Glacial) North Atlantic Deep Water. • The Iceland-Scotland basaltic sill acts as the major sediment supply during the last 400 ka, supporting the idea that convection areas in the Nordic Seas are always active. • The ISOW strength is marked by a well-defined glacial - interglacial variability, superimposed by millennial-scale variability, during glacial and interglacial periods. • The observed variations of ISOW intensity are in phase with deep-water ventilation changes further South in the North Atlantic basin on both orbital and millennial timescales.