Oceanographer: To substantially reduce the amount of carbon dioxide in Earth' s atmosphere, carbon dioxide should be captured and pumped deep into the oceans, where it would dissolve. The cool, dense water in ocean depths takes centuries to mix with the warmer water near the surface, so any carbon dioxide pumped deep into oceans would be trapped there for centuries.
It is the density of the water in the ocean depths that plays the main role in the trapping of the carbon dioxide is an assumption that the oceanographer's argument requires.