Two-Layered South Pacific Carbon Reservoirs and Deep-Water Export to the Atlantic During CO₂ Drawdowns Since the Last Interglacial

The rise in atmospheric CO₂ during the last deglaciation has been attributed to carbon release from the deep ocean, establishing the ocean interior as the dominant reservoir modulating glacial–interglacial CO₂ variability. In contrast, the processes by which carbon was sequestered in the ocean during periods of declining atmospheric CO₂ remain poorly constrained. In particular, three irreversible CO₂ drawdown events since the last interglacial (MIS 5d, MIS 4, and MIS 2) represent critical intervals for understanding how the ocean stored carbon over multi-millennial timescales. Here we reconstruct vertical profiles of carbonate ion concentration ([CO₃²⁻]) from deep-sea sediment records in the South Pacific, located at the junction of the Pacific and Atlantic deep-water pathways. Millennium-scale measurements of carbonate dissolution reveal distinct depth-dependent [CO₃²⁻] changes that correspond to variations in the Atlantic Meridional Overturning Circulation (AMOC) and the Antarctic Circumpolar Current (ACC). The results reveal that during periods of CO₂ drawdown, the South Pacific developed a pronounced two-layer structure, reflecting the contrasting influences of bathypelagic carbon accumulation and abyssal ventilation. By integrating our new records with existing datasets from the equatorial Pacific to Atlantic, we show that each of the three irreversible CO₂ drawdown events was characterized by carbon storage in different oceanic basins and depth ranges. These findings reveal that distinct circulation regimes of the AMOC and ACC governed deep-ocean carbon sequestration during glacial intensification, which advance our understanding of how the deep ocean regulates atmospheric CO₂.