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International audience; The evolution of intermediate circulation in the northern Indian Ocean since the last deglaciation has been reconstructed from two marine cores located at intermediate depths off the southern tip of India (MD77‐191) and in the northern Bay of Bengal (BoB) (MD77‐176). Benthic foraminiferal δ13C, seawater carbonate ion concentration ([CO32−]) estimated from the Sr/Ca, and paleotemperature reconstructed on the basis of the Mg/Li of aragonite benthic species Hoeglundina elegans were used to trace the evolution of past intermediate‐deepwater masses and to constrain ocean‐atmosphere exchanges during the two‐stage increase in atmospheric CO2 across the last deglaciation. The intermediate water [CO32−] was mainly affected by changes in the ocean alkalinity inventory, associated with the modulation of atmospheric CO2 on glacial‐interglacial timescales. Higher benthic foraminiferal δ13C, depleted [CO32−], and decreased benthic‐planktonic 14C age offsets at intermediate water depths suggest a release of deep‐sea CO2 to the atmosphere through the Antarctic Intermediate Water (AAIW) in the Southern Ocean during the 17–15.2 and 12.6–10.5 cal kyr BP time intervals. In addition, the decreased H. elegans Mg/Li record seems to reflect an increased contribution of cold water mass during the 17–15.2 and 12.6–11.9 cal kyr BP intervals and throughout the Holocene. In contrast, two warm events occurred in the 15–13.3 and 11–10.3 cal kyr BP time intervals. During the late Holocene, a decrease in the intermediate water [CO32−] indicates a contribution to atmospheric CO2 rise since 8 cal kyr BP, due to the depleted global ocean alkalinity and/or the variations in surface productivity (at least for MD77‐191).

Antarctic Intermediate Water (AAIW) plays important roles in the global climate system and the global ocean nutrient and carbon cycles. However, it is unclear how AAIW responds to global climate changes. In particular, neodymium isotopic composition (epsilon(Nd)) reconstructions from different locations from the tropical Atlantic have led to a debate on the relationship between northward penetration of AAIW into the tropical Atlantic and the Atlantic meridional overturning circulation (AMOC) variability during the last deglaciation. We resolve this controversy by studying the transient oceanic evolution during the last deglaciation using a neodymium-enabled ocean model. Our results suggest a coherent response of AAIW and AMOC: when AMOC weakens, the northward penetration and transport of AAIW decrease while its depth and thickness increase. Our study highlights that as part of the return flow of the North Atlantic Deep Water, the northward penetration of AAIW in the Atlantic is determined predominately by AMOC intensity. Moreover, the inconsistency among different tropical Atlantic epsilon(Nd) reconstructions is reconciled by considering their corresponding core locations and depths, which were influenced by different water masses in the past. The very radiogenic water from the bottom of the Gulf of Mexico and the Caribbean Sea, which was previously overlooked in the interpretations of deglacial epsilon(Nd) variability, can be transported to shallow layers during active AMOC and modulates epsilon(Nd) in the tropical Atlantic. Changes in the AAIW core depth must also be considered. Thus, interpretation of epsilon(Nd) reconstructions from the tropical Atlantic is more complicated than suggested in previous studies.