I previously reported the possibility of sinking excess carbon dioxide to the bottom of the ocean. Now I've been reading many other papers claiming that the same could be done directly in the ocean.
Apparently iron is the key ingredient missing in most sea plants. That is, lack of iron is slowing their growth. So if we add iron to the middle of the ocean, there would be a wild burst of growth there. That growth, when it dies, would sink to the bottom, locking the carbon there for a very long period of time.
The reports aren't clear if the iron they need to add would be shavings (which would sink pretty quickly), or iron-rich biological material (such a blood from a slaughterhouse), but supposedly, a fleet of tankers could single-handedly cause an ice age.