Seawater carbonate ion-δ¹³C systematics and application to glacial-interglacial North Atlantic ocean circulation

Seawater carbonate ion and δ¹³C are affected by many processes including biology, air-sea exchange, alkalinity change, and mixing between different water masses. Study of modern ocean data shows that deep ocean carbonate ion and δ¹³C of dissolved inorganic carbon can be used together as useful tracers for deep water mass reconstructions in the past. We present records of deep water carbonate ion concentration ([CO₃^2-]) changes of the North Atlantic Ocean water column since the last glacial, quantitatively reconstructed using benthic foraminiferal boron/calcium (B/Ca) ratios. Records from six cores over 1 to 4 km reveal that the carbonate chemistry of the glacial North Atlantic was more stratified than the modern ocean, with higher [CO₃^2-] by ~ 20-30 μmol kg^{- 1} at 1-2 km and lower [CO₃^2-] by ~ 20 μmol kg^{- 1} at sites deeper than 3.5 km, producing an 800 m glacial shoaling of calcite saturation horizon. Comparison with benthic foraminiferal δ¹³C and e{open}Nd of Fe-Mn oxide leachates shows that the deep glacial waters with low-[CO₃^2-] are consistent with a Southern Ocean source, while those with high-[CO₃^2-] but distinct δ¹³C chemistry were supplied by two endemic sources, one being the Norwegian-Greenland Sea (NGS). Our carbonate ion results suggest that the glacial boundary between north and south sourced deep waters is at ~2.8 km, significantly deeper than ~ 2.3 km estimated from benthic foraminiferal δ¹³C. Weakened surface compensation inflow to the NGS promoted cooling and continental ice growth at north high-latitude, and a deeper boundary may reduce atmospheric CO₂ sequestration in the deep Atlantic, implying a greater role of other parts of the ocean.