Fig. 4: Schematic illustrations of the changes in the Atlantic basin across the MPT during.

A Glacials and IRD events before MIS 26 (960 ka), and (B), (C) two possible scenarios for glacials and IRD events after MIS 26 (960 ka). The glacials before MIS 26 (1069–960 ka) are characterized by a lower NADW production in the high-latitude North Atlantic, but without dramatics reductions in BPWO (as observed in BPWO records from Sites U1314 and U1385). One exception is MIS 30, which is characterized by a moderate reductions in bottom-ocean O₂²³. The post-MIS 26 period is characterized by a much larger ice-volume in both polar regions^7,15, sea-ice and surface ocean cooling^28,88, and an unprecedent reduction in BPWO as shown by Site U1314 and Site U1385 proxies records which reflects the weakening of the AMOC¹⁹. The main feature is the shoaling in the boundary between AABW and NADW, and the strengthening in mid-depth circulation via formation of glacial North Atlantic intermediate waters (GNAIW)⁴⁸. Two potential scenarios are proposed: (B) Northward intrusion of southern source waters, as suggested by benthic carbon isotope records^6,45,47 from the Atlantic basin for the MPT; (C) Uninterrupted but weak deep-convection in the North Atlantic, with NADW formed in isolation from the atmosphere (under sea-ice) and/or with longer residence time, following recent models of thermohaline circulation in the high-latitude North Atlantic proposed for the LGM^55,56. Small blue arrows in (C) denote deep-water convection below sea-ice. Large Arrows indicate the major deep and intermediate ocean water mass flows, with thin arrows associated with NADW and AABW representing an inferred lower flux of these water masses during glacials.