Fig. 3: Deriving PO₄* and [CO₃^2-]_as at site MD97-2106.

Upper panel shows changes in [PO₄^3-] (a) and [O₂]/175 (b) relative to the LGM averages. PO₄* (c) is determined by the sum of [PO₄^3-] and [O₂]/175. Lower panel shows reconstructed [CO₃^2-] (d) alongside expected changes in [CO₃^2-] relative to the LGM values (e), due to temperature-salinity-pressure variations (grey curve), DIC-alkalinity redistributions linked to [PO₄^3-] variations (green curve), and temporal global ocean alkalinity-DIC changes linked to CaCO₃ input/output imbalance (orange curve). All curves shown in (e) assume no air-sea CO₂ exchange. f calculated [CO₃^2-]_as. In (c–f) empty circles with error bars on the y-axes show the averages and ±1 standard deviations for the late Holocene data. Envelopes associated with curves represent ±1σ error ranges incorporating uncertainties from analytical measurements, calibrations, and the age model. The deglacial PO₄* pattern is dominantly driven by [O₂] changes, while millennial-timescale [CO₃^2-]_as variabilities are mainly driven by changes in [CO₃^2-] with secondary contributions from DIC-alkalinity variations linked to [PO₄^3-] fluctuations. Grey vertical shadings highlight the Younger Dryas (YD) and Heinrich Stadial 1 (HS1), which are interrupted by the Antarctic Cold Reversal (ACR). The vertical dashed line marks the boundary between the early and late stages of HS1.