Two-layered deep-ocean carbon storage and Pacific–Atlantic redistribution drive glacial CO₂ drawdowns

Atmospheric CO₂ rise during the last deglaciation is attributed to deep-ocean carbon release, yet the mechanisms driving basin-scale carbon sequestration during CO₂ decline remain unclear. Three late-glacial CO₂ drawdowns since the last interglacial (MIS5d-b, MIS4, and MIS2) provide key intervals to resolve these processes. Here we reconstruct vertical profiles of carbonate ion concentrations ([CO₃²⁻]) in the South Pacific at a critical Antarctic Circumpolar Current (ACC) gateway linking Pacific and Atlantic deep waters, using a µCT-based carbonate dissolution proxy calibrated to bottom-water [CO₃²⁻]. During CO₂ drawdown intervals, our reconstruction uncovers the emergence of a pronounced two-layer structure, marked by a 10–15 µmol kg⁻¹ vertical [CO₃²⁻] gradient, indicating upper bathypelagic carbon accumulation overlying well-ventilated lower waters. Integration with equatorial Pacific and Atlantic records reveals basin- and depth-specific carbon storage, demonstrating that Pacific–Atlantic carbon redistribution regulated by Atlantic Meridional Overturning Circulation (AMOC)–ACC can account for multi-millennial atmospheric CO₂ drawdown.