What Did John Martin Mean When He Said "Give Me

What did John Martin mean when he said "Give me half a tankerload of iron, and I'll give you an Ice Age"?

A) Iron added to ocean water decreases NPP, and that decrease would lead to more carbon dioxide being taken up by phytoplankton. That would lead to the removal of carbon dioxide from the atmosphere, thus reducing the greenhouse effect.
B) Iron added to ocean water increases NPP, and that increase would lead to less carbon dioxide being taken up by phytoplankton. With phytoplankton taking up less carbon dioxide, more carbon dioxide would be removed from the atmosphere, thus reducing the greenhouse effect.
C) Iron added to ocean water increases NPP, and that increase would lead to more carbon dioxide being taken up by phytoplankton. That would lead to the removal of carbon dioxide from the atmosphere, thus reducing the greenhouse effect.
D) Iron added to ocean water changes the specific heat of water, such that water can hold less heat. This leads to subsequent cooling of the atmosphere.