The North Atlantic Ocean underwent an abrupt temperature increase of 9 °C at high latitudes within a couple of decades during the transition from Heinrich event 1 (H1) to the Bølling warm event, but the mechanism responsible for this warming remains uncertain. Here we address this issue, presenting high-resolution last deglaciation planktic and benthic foraminiferal records of temperature and oxygen isotopic composition of seawater (δ18Osw) for the subtropical South Atlantic. We identify a warming of ~6.5 °C and an increase in δ18Osw of 1.2‰ at the permanent thermocline during the transition, and a simultaneous warming of ~3.5 °C with no significant change in δ18Osw at intermediate depths. Most of the warming can be explained by tilting the South Atlantic east-west isopycnals from a flattened toward a steepened position associated with a collapsed (H1) and strong (Bølling) Atlantic meridional overturning circulation (AMOC). However, this zonal seesaw explains an increase of just 0.3‰ in permanent thermocline δ18Osw. Considering that δ18Osw at the South Atlantic permanent thermocline is strongly influenced by the inflow of salty Indian Ocean upper waters, we suggest that a strengthening in the Agulhas leakage took place at the transition from H1 to the Bølling, and was responsible for the change in δ18Osw recorded in our site. Our records high-light the important role played by Indian-Atlantic interocean exchange as the trigger for the resumption of the AMOC and the Bølling warm event.

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