Fucking around with the ocean on a scale that's going to move the needle on CO2 in the atmosphere has a very high probability of serious unintended consequences.
Iron particles won't replicate, but the chemistry of the ocean could change dramatically. We don't know what happens at scale, and we probably don't want to find out.
Not doing anything has a certainty of turning the ocean to a mild acid, with disastrous effects that are already becoming visible in coral bleaching and shellfish die offs. And carbon that is already in the atmosphere and not yet dissolved is only going to make it worse.
We are at the point of choosing between what disasters we will suffer, rather than figuring out how to not have a disaster.
Why are we so concerned about ocean acidification from CO2, when past CO2 levels were far higher than now and all of the shellfish species that currently exist lived back then too? Wouldn't it be a good idea to consider the massive quantities of known toxins we pump into the oceans rather than fixating on a red herring?
Your question assumes that the oceans had to have been acidic because CO2 was high. This assumption is wrong.
You don't get acidification as long as CO2 levels change slowly enough that it mixes down to the bottom of the ocean and then gets buffered by calcium bicarbonate being dissolved there and mixing back to the top. But this mixing takes place on the scale of a thousand years. This is no big deal for CO2 level changes taking place over geological time. But it doesn't help shellfish with sudden increases of CO2 taking place on a scale of decades or centuries.
> Bivalves grown under near preindustrial CO2 levels displayed thicker, more robust shells than individuals grown at present CO2 concentrations, whereas bivalves exposed to CO2 levels expected later this century had shells that were malformed and eroded. These results suggest that the ocean acidification that has occurred during the past two centuries may be inhibiting the development and survival of larval shellfish and contributing to global declines of some bivalve populations.
i'm not sure where the claim "all of the shellfish species that currently exist lived back then too" is coming from, although it is correct that CO2 was higher in the past (https://www.skepticalscience.com/co2-higher-in-past.htm)
Technically, seeding the ocean with iron particles is a suspension, not a solution. Luckily, the common technique is to use Iron(II) sulfate, which mixed with water does create a solution!
