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

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 degrees 29.90 degrees N/17 degrees 56.88 degrees 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 similar to 2 degrees C persisted for similar to 5.4 Kyr coincident with the weakest stage of AMOC; and second in the Younger Dryas, when bottom water temperatures >4 degrees C lasted similar to 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. Plain Language Summary Anthropogenic activity affects earth-atmosphere energy balance enhancing climate change in the last decades. The ocean plays a key role in this balance, by taking up to 90% of the excess heat from the atmosphere and redistributing it globally though the Atlantic Meridional Overturning Circulation (AMOC). For the 21st century, a possible 34%-45% AMOC reduction has been hypothesized, raising concern on its effects on ocean heat uptake and climate change. To contribute to the understanding of these possible effects, we reconstructed bottom water temperatures changes for eastern North Atlantic deep waters over the last 46,000 years, including two periods with a significantly reduced AMOC: (a) Heinrich Stadial 1 (18,200-14,900 years ago) and (b) the Younger Dryas (12,800-11,700 years ago). Our results suggest that with a weak AMOC, Atlantic intermediate waters warm as ventilation decreases, at the same time this heat stops being transferred to the deep eastern North Atlantic for thousands of years. The implications of these processes for global warming still need to be investigated.