Rapid increase in Antarctic intermediate water ventilation related to past Atlantic meridional overturning circulation weakening

Abstract Southeast Pacific intermediate waters feed the global thermocline and thus can widely influence ocean biogeochemical and physical properties with consequences for climate and ecosystems. Despite this importance, the sensitivity of intermediate waters to past and current climate change remains inadequately understood. Here we reconstruct variability of Antarctic Intermediate Water during Antarctic Isotope Maximum event 8 (37–39.4 ka) using benthic foraminiferal stable isotopes from Ocean Drilling Program Site 1233. At 39.4 ka, bottom waters warmed, oxygen concentrations rose sharply, and carbon isotope values increased, indicating enhanced ventilation and nutrient depletion. These changes are consistent with the thermocline deepening and nutrient redistribution found in models in response to a weakened Atlantic Meridional Overturning Circulation. Intensification of Antarctic Intermediate Water ventilation during rapid Southern Hemisphere warming helps to constrain carbon dioxide rise hypotheses and identifies intermediate waters as sensitive modulators of thermocline biogeochemistry.