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 ([CO32−]) 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 [CO32−] by ~ 20–30 μmol kg− 1 at 1–2 km and lower [CO32−] 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 ɛNd of Fe–Mn oxide leachates shows that the deep glacial waters with low-[CO32−] are consistent with a Southern Ocean source, while those with high-[CO32−] 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.