1.Supertankers would spread millions of tons of iron over the ocean surface.
2.The iron stimulates growth of algae which consume carbon dioxide from the ocean surface as they grow. 
3.When the algae and the organisms that consume them die, they sink to the sea floor, taking the carbon with them.
4.The ocean draws more carbon dioxide from the atmosphere to replace what the algae took from its surface.
These techniques would involve removing the CO2 as itŐs emitted from a power plant and either injecting it directly into the deep ocean or burying it deep in the ground.