New evidence for slowdowns of the Atlantic overturning circulation in response to iceberg discharges

Climate archives have revealed that rapid climate changes took place during the last glacial period, whereby cold intervals in the North Atlantic region were associated with Atlantic Meridional Overturning Circulation (AMOC) slowdowns1–6. Some of these cold intervals were also characterized by ice rafted debris (IRD) layers in North Atlantic sediments, stemming from iceberg discharges from the northern hemisphere ice sheets7–9. The IRD layers were first interpreted as the imprint of intense melt water input by icebergs to the surface ocean that hindered deep-water formation and led to the observed AMOC slowdowns. However, subsequent studies dismissed iceberg discharges as a cause of AMOC slowdown on the basis of observations from marine cores from the Reykjanes Ridge region, indicating that surface cooling systematically precedes the arrival of IRD10,11. Here we analyse well-resolved IRD and reconstructed surface ocean temperature records from glacial North Atlantic sediments, together with numerical simulations. We show that in vast portions of the North Atlantic, there is no systematic lead of surface cooling over the arrival of IRD. Our results therefore invalidate the arguments made that AMOC slowdowns were not triggered by iceberg discharges and show that surface freshening is a plausible physical trigger of AMOC slowdown.