Late Slowdown of Atlantic Meridional Overturning Circulation During the Last Glacial Inception: New Constraints From Sedimentary (231Pa/230Th)

The Atlantic Meridional Overturning Circulation (AMOC) response to northern hemisphere insolation forcing and North Atlantic surface hydrology is investigated on the period covering the Last Interglacial to glacial Marine Isotope Stage (MIS) 4, with emphasis on transitions from interstadials to stadials. We provide the first sedimentary (Pa/Th) profiles over the 130 to 60ka interval, which present significant changes despite the relatively short half-life of 231Pa (32.1 ka). They are measured in two cores from the western (SU90-11, 3645m) and eastern (MD01-2446, 3547m) basins of the North Atlantic. The (Pa/Th) profiles display a high degree of coherency with available benthic δ13C and foraminifera summer SST profiles from nearby reference sediment cores. They do not show any correlation with diatom valves flux and are therefore interpreted in terms of changes in AMOC export. Our results show an AMOC drop in parallel with the enhancement of ice rafted detritus, marking the increased growth and calving of Northern ice-sheets. AMOC remained vigorous well after each decrease of Northern summer insolation within MIS 5 and is likely driven by the seasonal meridional insolation gradient. The associated transport of heat and moisture from the tropics towards the Northern latitudes favoured ice-sheets growth. In return, the ice-sheets dynamics impacted the AMOC during periods of iceberg discharge and enhanced freshwater input in the North Atlantic. The associated slowdowns of AMOC export are estimated to be about 40%±15%. Following these slowdowns, the AMOC quickly recovered its interstadial vigour. The lag of the AMOC response to decreased Northern summer insolation reduces as the continental ice-sheets increase in size, from about 15ka at MIS 5.5--5.4 transition, 10ka at MIS 5.3--5.2 transition to about 3ka at MIS 5.1--4 transition.