Shifts in Antarctic Intermediate Water properties coincide with atmospheric CO₂ rise across the Mid-Brunhes Event

Antarctic Intermediate Water (AAIW) is key to the global carbon cycle, yet its influence on past atmospheric CO₂ changes remains unclear. Using multiproxy reconstructions from the data-poor Pacific sector of the Southern Ocean, we examine interglacial AAIW variability in its source region across the Mid-Brunhes Event (MBE), a major CO₂ transition. While surface temperatures remained stable over 600 thousand years, post-MBE AAIW became warmer and saltier, possibly due to reduced iceberg-derived freshwater input. In contrast, colder, fresher pre-MBE AAIW and enhanced thermal stratification may have promoted greater CO₂ uptake and storage. The post-MBE declining sequestration capacity of AAIW, coinciding with rising atmospheric CO₂, suggests intermediate waters played a critical role in modulating CO₂, challenging the view that changes in bottom-water processes alone controlled this key climatic transition.