Stagnant North Atlantic Deep Water Heat Uptake With Reduced Atlantic Meridional Overturning Circulation During the Last Deglaciation

Abstract

Atlantic Meridional Overturning Circulation (AMOC) plays a major role in the climate system by modulating the depth and rate of oceanic heat storage. Some climate simulations suggest that reduced AMOC decreases bottom water ventilation and that the heat absorbed by the ocean starts to mix downwards, warming Atlantic intermediate waters. This has been corroborated for the western North Atlantic by benthic foraminifera geochemical records from periods of reduced AMOC during the last deglaciation. However, the deep-water response remains poorly constrained, and the lack of direct paleotemperature reconstructions limits our understanding about the effects of reduced circulation on ocean heat uptake. We present a new reconstruction of bottom water temperatures from core GeoB9508-5 (2,384 m water depth, 15°29.90°N/17°56.88°W) off the northwestern African Margin. Our paleotemperature record, based on Uvigerina spp. Mg/Ca, shows two episodes of intense transient deep water warming in times of decreasing overturning circulation, followed by long periods of heat uptake stagnation. First, during AMOC slowdown in the Heinrich stadial 1, when paleotemperatures of ∼2°C persisted for ∼5.4 Kyr coincident with the weakest stage of AMOC; and second in the Younger Dryas, when bottom water temperatures >4°C lasted ∼2.5 Kyr during a less intense AMOC decline. This suggests a stagnation of deep-water heat uptake in the deep NE Atlantic possibly linked to a reduced downward advection of heat during times of a reduced AMOC, supporting the hypothesis that AMOC strength sets the depth of oceanic heat storage in the North Atlantic.