No its not a joke there's multiple processes one of which is the algae fertilizer. It's a very efficient carbon sink.
It looks like a joke, so it was downvoted when I saw it. It remembered some project to add iron to the sea, so I hoped that it was a serious proposal.
Try to avoid onliners, it's very easy to misinterpret them. (Bonus points for a link to Wikipedia or somewhere with more details.)
(Is it posible to use raw mineral ore? The production of steel/iron use a lot of carbon.)