Plant life in the oceans is mainly limited by minerals, especially iron. That's why the oceans aren't green and slimy. Dumping iron powder in will usually cause an algae bloom, which eventually die and take the carbon they fixed to the bottom, so seeding the oceans with iron is a potential way of reducing CO2 in the atmosphere. Of course, like any large-scale meddling, there will be collateral damage.
> will usually cause an algae bloom, which eventually die and take the carbon they fixed to the bottom
It works so well that this could even be how the current "icehouse earth" conditions got started in the first place. The Azolla bloom of 49 ma dropped atmospheric CO2 levels from 3500ppm to 650ppm in less than 1 my, and CO2 has been in that lower range ever since.
