The main point of iron fertilization is to make plankton and other organisms grow in the ocean so that some will sink to the bottom and capture carbon that way, not so much promote reflection. Seeding the atmosphere with sulfur is one approach to that, cloud brightening another.

One unintended consequence of iron fertilization could be the promotion of growth of toxic plankton that add neurotoxins to the food chain (e.g. “Red Tide”) but that’s not the only one.

Another interesting aspect of this is that the UN Law of Sea forbids this. Most countries with a coastline are signatories with the notable exception of the US:

https://en.wikipedia.org/wiki/List_of_parties_to_the_United_...

Notably if you were trying to do this, most countries, like Japan or Australia, could refuse the use of their ports.