The deep waters of the Pacific and Indian Oceans are a mixture of North Atlantic Deep Water (NADW) and Southern Ocean Ventilated Water (SOVW), which combine in the deep Southern Ocean (the ring of ocean that encircles Antarctica and extends north to South America, Africa, and Australia). The product of this combination, Circumpolar Deep Water (CPDW), then flows north into the deep Pacific and Indian basins. One might expect that the carbonate ion concentrations of the deep Pacific and Indian Oceans would be between those of NADW and SOVW, but they are not. Instead they exhibit an apparent deficiency in carbonate ion that cannot be explained by chemical or biological processes.
Broecker and Sutherland consider this dilemma by first noting that CPDW displays the same carbonate deficiency as the deep waters of the Pacific and Indian Oceans. This means, then, that it is in the Southern Ocean that the carbonate deficiency originates. They suggest that the cause of the deficit could be that deep-water formation in the Southern Ocean was much stronger during the Little Ice Age (∼1350 to 1880 A.D.) than it is now, and that the carbonate ion content of that water was lower as well. Therefore, the deep Pacific and Indian Oceans that exist today may have been formed with a component of Southern Ocean deep water unlike the SOVW that is being made at present. — HJS
Geochem. Geophys. Geosys. 1 (2000).