The geometry and ventilation of upper deep-water masses in the Pacific sector of the Southern Ocean are critical to glacial carbon storage, yet their evolution since the Last Glacial Maximum remains poorly constrained. Here, we present the foraminiferal neodymium isotope composition (εNd) from core MD97-2115 (eastern Chatham Rise, 2160 m water depth), which is located at the present-day boundary between Upper Circumpolar Deep Water (UCDW) and modified North Pacific Deep Water (mNPDW) in the Southwest Pacific, to reconstruct the upper deep-water mass changes since 30 ka. Combining quantitative binary mixing calculations with vertical εNd gradients relative to Lower Circumpolar Deep Water, we suggest substantial shifts in the geometry and mixing of UCDW and mNPDW since 30 ka. The variation phases include reduced UCDW influence during 30–20 ka, enhanced UCDW contribution between 20 and 14.3 ka, slightly decreased UCDW during 14.3–11.4 ka, and stable Holocene conditions. The glacial dominance of mNPDW is consistent with basin-wide evidence for enhanced storage of respired carbon in Pacific-sourced deep waters, likely contributing to the maintenance of low atmospheric CO2 during the glacial period. Crucially, the early rapid increase in UCDW at ∼20 ka preceded the penetration of North Atlantic-sourced water into Southern Ocean and the deglacial CO2 increase by several millennia, implying a critical role for Antarctic and Southern Ocean processes in establishing the oceanic precondition for glacial termination. During the last deglacial period, the decoupling between our reconstruction and atmospheric CO2 evolution suggests that water-mass geometry alone is insufficient to drive the full deglacial CO2 increase, pointing to additional roles for processes such as regional outgassing and variability in air–sea gas exchange efficiency.
