One aspect these iron fertilization experiments have ignored is albedo. Leaving any sequestration on the table, and just focussing on keeping the albedo of equatorial oceans low for as long as possible has the real potential to avoid absorbing massive amounts of solar heat into the system. That doesn't solve the co2 problem, but because you can negate the feedback loop aspect of the problem, and perhaps even make natural sequestration occur faster if you lower temperature enough. One thing I haven't been able to figure out from the data from the experiments, is how long these blooms last. The tonnage of fertilizer and the size of the bloom, yes. Also, the experiments were done in eddies mostly to isolate the effects. So an open equatorial ocean experiment would be needed. Along with better local temperature monitoring. Some satellite imagery to verify albedo would be a plus. As far as ballpark numbers, if the bloom stays equatorial, you would need about 1.5e9 USD per however long a bloom lasts in order to maintain an albedo that would initiate snowball. That is scaling linearly up from the area affected by previous experiments vs their costs.