Deglacial ocean density de-stratification with a weaker Atlantic Meridional Overturning Circulation

Atmospheric heat and carbon uptake and storage by the ocean are controlled by seawater stratification, which is also linked to Atlantic Meridional Overturning Circulation (AMOC) through ocean heat distribution that can modify density stratification. The effects of a potential weakening of the AMOC on ocean stratification, and therefore on heat uptake and storage, remain an open question. To gain insight into these dynamics, we used marine sedimentary archives of the last deglaciation (last 27000 years) to reconstruct temperatures at intermediate (GeoB9512-5, 793 m water depth) and deep (GeoB9508-5) water masses of the eastern Atlantic off the coast of Senegal (northwestern Africa). During this time, marked changes in AMOC strength took place: the Last Glacial Maximum (LGM, 23,000 – 19,000 years ago), a time of shallower meridional overturning; and the Heinrich Stadial 1 (HS1, 18,200–14,900 years ago) and Younger Dryas (YD, 12,800–11,700 years ago), when AMOC was weaker than today. Our benthic foraminifera-based Mg/Ca (seawater temperature) and δ18O (ocean density) show that a persistently shallow and strong (LGM) or weak (HS1 and YD) meridional overturning led to a mid-depth warming at the same time deep-ocean heat uptake was paused, leading to a strong density stratification in the Atlantic. These results are compatible with previous temperature reconstruction across the tropical and north Atlantic, and also show that with a Holocene AMOC strengthening, mid-depth cooling and resumed deep-ocean heat uptake resulted in a weaker stratification. Our findings show that the AMOC state sets the depth of heat storage and that the depth of the upper AMOC cell is tightly related to deep ocean stratification.