"Give me a half a tanker of iron and I will give you another ice age" - John Martin 1991

A bit of an overstatement, but nevertheless.  Ocean iron fertilization (OIF) is a CDR technique which involves depositing large amounts of iron into the surface ocean as a way of enhancing the biological pump.

For those who are unfamiliar with the concept of the biological pump; in essence it's the process of phytoplankton uptaking dissolved inorganic carbon during photosynthesis, and some of this fixed carbon being deposited to the deep ocean as dissolved organic matter (DOM).  However, only 1% of all carbon taken up at the surface is actually eventually sequestered into ocean sediments due to other processes acting on it in both the surface and deeper ocean which turns it back into dissolved inorganic carbon.

In 1990 John Martin hypothesised that by adding iron to nutrient-rich but low productive (HNLC) ocean regions, it would stimulate phytoplankton blooms, which would drawdown significant amounts of atmospheric carbon dioxide into the oceans and increase the rate at which carbon is sequestered into the deep ocean.  HNLC regions were identified in the northern and equatorial Pacific and the Southern Ocean, and since 1993 12 experiments have occurred in these regions.

(WHOI 2008)

Ideally, the Pacific is easier to work in due to calm seas, lots of sunlight and warmth which stimulates algal growth.  However, the Southern Ocean contains 3-5 times higher nutrient levels - although lacks warmth and sunlight for many parts of the year.
It is worth noting these experiments were only on a small scale, but results showed OIF did indeed stimulate algal blooms temporarily (IPCC).  However, the experiments are somewhat inconclusive in the way many important questions were raised but remained unanswered.  Most importantly, it was difficult to tell exactly how much extra carbon was sequestered as a result of enhanced algal blooms, and the side-effects of such an experiment were a cause for concern (although no ecological impacts were observed during these experiments, but they were only on a small scale (WHOI 2008)). 

Potential negative consequences of OIF

• Shift in phytoplankton species, from small phytoplanktonic species to a diatom dominated state (Greenpeace), resulting in possible changes in the food web.
• Nutrient depletion in surrounding waters (WHOI 2008).
• Deoxygenation of surface waters; which may cause a negative effects on other marine organisms.
• Development of harmful algal blooms.

Considering this 2 decades on OIF still remains a controversial idea.  Despite the possible drawbacks and hazards associated with this method, many scientists and (increasingly) private companies back the idea and are keen to develop more experiments - this time on a much larger scale in the hope of obtaining more clear results.  These parties argue our present knowledge of the ocean is somewhat unclear; and further experimentation is in everyone's interest.  Additionally; future proposed projects are still on too much of a small scale to cause lasting environmental damage.

In conclusion to using OIF as a CRT then, it is clear it is not as effective as early models predicted, and poses many difficult questions.  A major drawback is the fact it is so difficult to measure not only how much more carbon is actually being removed from the atmosphere, but how much of this is actually being sequestered.  Storing carbon in larger organisms or algae in the middle-depth waters is potentially a short-term solution to employ whilst better technologies are developed, but will only remain here for a few decades at most.  Obviously this means OIF is not suitable for being considered as a long-term technique to stopping environmental change (Davis 2008).  The fact only 12 experiments have been employed in over 2 decades suggests lack of interest in the method, and it is possibly difficult to obtain funding for something which does not have guaranteed success.  The technology has not yet seemed to have advanced very much since John Martin's declaration of the iron hypothesis in 1991, so maybe it is time to develop more sophisticated and advanced techniques of CDR?

The future....

More experiments are being considered by both scientists and private companies; this time on a bigger scale with around 100 tons of iron involved (WHOI 2008).  Certainly better monitoring technologies are required to measure and improve the knowledge of sequestration.

“You might make some of the ocean greener by iron enrichment, but you’re going to make a lot of the ocean bluer.” - Robert Anderson.  Maybe OIF will have to be considered as a trade-off!