"Give me a half a tanker of iron and I'll give you the next ice age."
-- Oceanographer John Martin (Joking), in a 1991 speech.
Iron fertilization is a theoretical but not promising type of geoengineering that attempts to stimulate carbon sequestration in the ocean by way of algal blooms.
Because iron is insoluble in salt water, and because it is a necessary nutrient for photosynthesis in plants, adding iron to the sea will stimulate an algal bloom. The theory is that plankton will absorb atmospheric carbon dioxide while they grow, and upon their death will sink to the ocean floor, where the carbon in their corpses will be ensconced safely in the collecting sediment of the sea-floor.
The general mechanics of this process have been known for decades, but what really set things off was the 1991 eruption of Mount Pinatubo in the Philippines. It ejected about ten cubic kilometers of material, much of which fell into the oceans as ash. This included perhaps 18,000 cubic meters of iron dust, and did indeed stimulate algal blooms; over the next year the expected increase in atmospheric carbon dioxide fell significantly short, by the greatest amount seen in the 35-year history of the NOAA Mauna Loa Earth System Research Laboratory. This was accompanied by an increase in atmospheric oxygen equal to what you would expect if the carbon deficit had been caused by an oceanic algal bloom. (It is worth noting that the eruption of Mount Pinatubo also cooled the Earth by 0.9 degrees Fahrenheit by releasing light-colored sulfur particles and other aerosols into the atmosphere, reflecting the sun's rays back into space; this is called atmospheric seeding, and is another possible method of geoengineering).
Experimentation has had mixed results. It appears that the basic theory is sound enough, but getting things to work in practice is difficult. For iron fertilization to be effective at reducing atmospheric carbon, it has to take into account the other nutrients present (diatoms are best at sequestering carbon, but require silicic acid), the probability that the dead phytoplankton will rest in situ and not be eaten -- while at the same time, hopefully, keeping the existing ecosystems healthy. We also, obviously, have limited data on the long term effects.
And on the flip side, iron fertilization is cheap and easy enough that private citizens can do it themselves. The Haida Salmon Restoration Corp. recently made international news when they dumped "100 tonnes of an iron-rich dirt-like material" into the waters off of British Columbia's north coast in order to produce a plankton bloom in order to feed local salmon, in an attempt to increase the salmon populations. This was funded, in part, by American businessman Russ George, founder of the geoengineering firm Planktos Inc. He has undertaken similar projects in the past, in the hopes that he may help reverse climate change. Critics point out that we don't know what these sorts of actions might be doing to the ecosystem, and that the risk is not worth the payoff. They have a point -- the recent dump in British Columbia is considerably larger than any formal experiment ever undertaken.
This isn't completely uncharted territory, however. Natural iron fertilization occurs when an upwelling of nutrient-rich water brings iron to the surface of the ocean. This may happen when ocean currents come up against a seamount or a shoal; rivers feeding into the sea, dust-rich winds, and volcanic eruptions may also be the cause of iron fertilization. Regular sources of iron fertilization create stable and rich ecosystems; it is not the existence of iron that is the problem, only the introduction of large amounts of it suddenly into new areas. Because there are a number of naturally iron-rich zones in the oceans, most iron fertilization experiments take place in High Nitrate, Low Chlorophyll (HNLC) zones.
Wikipedia: Iron Fertilization
Wikipedia: Iron Hypothesis
Metal Ions in Biological Systems by Astrid Sigel
Duke University: Nicholas School of the Environment: Assessing the Pros and Cons of Geoengineering to Fight Climate Change
AWI: LOHAFEX: An Indo-German iron fertilization experiment
Huffington Post: Iron Sulfate Haida Gwaii Dump Defended
CBC News: Iron fertilization project stirs West Coast controversy
Wikipedia: Russ George