I am currently doing a project for a course dealing with Sea CO2 management. I manged to receive a message from the Secretary of the London Convention. Here it is, for your own pleasure 
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"Concerning your question about the role of the London Convention/Protocol in case of capture CO2 from the atmosphere you are advised as follows. I have linked this response with the use of iron to capture CO2.
Sequestration of CO2 from fossil fuel production and use has been discussed briefly under the London Convention a couple of years ago, but without definitive conclusions. You may be aware that the main discussion about CO2 concentrations and influence on climate change takes place in the UNFCCC framework (Kyoto Protocol). The Scientific Group under the London Convention has a “watching brief” to follow the discussions in other fora and the CO2 experiments that are being planned and or executed. Iron fertilization has not yet been discussed, so the advice I give you is my opinion as the secretary for the London Convention.
One might argue that putting iron at sea in large quantities to capture CO2 should not be seen as dumping but as “placement of matter for a purpose other than the mere disposal thereof, provided that such placement is not contrary to the aims of this Convention” under its Article III(1)(b)(ii), or the equivalent provision in Article 1.4.2.2 under the Protocol.
But this assumes that any impact will be limited and acceptable for the greater good of mitigation of climate change and in order to buy time to switch to more sustainable forms of energy generation and use.
Even if this is the case, for massive disposal/sequestration of CO2 in the oceans, the 1996 Protocol will need to be amended, as CO2 does not fit in any of the 7 exempt categories under the full dumping prohibition of Article 4.1.1 of the Protocol.
I understand that, maybe for this reason, attention is also being given to CO2 capture and storage in geological formations rather than its sequestration in the oceans itself, presumably because the degree of controlling any operations is, at least conceptually, much higher. If this practice would be acceptable, the Protocol may also need an amendment here, as the definition of dumping includes the storage of wastes in the sea-bed and the subsoil thereof (Article 1.4.1.3)."
Obviously, because of the possible environnemental impacts, I believed the London Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter, 1972 and protocol, 1996 would prevent any thoughts of CO2 dumping and iron fertilization especially because of Article I of the London Convention
And, of course, I thought SOCA would be screaming bloody murder on this, but I guess the UNFCCC will treat the others agencies like a catholic priest can do to a choir-boy.