Also, is the solution just dissolved iron sulfate or also dissolved silica ?
Sorry for the pun in the title. It is a reference to one of the hives on twitter — the nuclear energy enthusiasts who note the large fraction of zero carbon electricity currently produced by nuclear reactors.*
Another approach to dealing with global warming is carbon capture. There is industrial scale carbon capture technology (which makes nuclear energy look cost competitive) but I would suggest sticking to photosynthesis — in particular by diatoms.
Marine diatoms are an algae which produces a shell made of glass. They are pretty much undigestible and, when they die, end up on the bottom of the ocean as sedimentary permanently captured carbon. The growth of marine algae is limited by lack of small amounts or required minerals (which is why sea including glacial melt is rich and the crystal clear water (say Mediterranean or Caribbean) far from glaciers is poor). In particular, marine algae need iron. This means that moderate amounts of Iron ions (from dissolved Iron sulfate) cause marine algal blooms. A problem (noted by a commenter here long ago) is that fish eat the algae and turn the fixed carbon back to C02.
The approach only works if the algae are hard to digest, that is diatoms.
This makes me think two thoughts. First it might be useful to release diatoms along with the iron sulfate. This would give them the edge on more appetizing algae. Another is that the diatoms also require desolved silica (to make the glass shells).
The fertilize the ocean approach works in areas with disolved silica so the diatoms are limited by Iron not silica. Also silica can be released along with the iron (again to give diatoms an edge). A problem with the last thought is that diatoms contain a lot of silicate (aka glass) compared to Iron so the amount of silica needed might be huge and costly.
I suppose a concluding thought is that we should consider where carbon currently is. Most of it is not in the atmosphere, coal, methane, petroleum, or wood. It is mostly in limestone and was mostly fixed in the sea not on land.
*Having distracted myself, I note that the main troublemakers for nuclear energy enthusiasts are no longer confused greens but rather green eyeshade wearing accountants, who note that nuclear energy is not profitable (there is one nuclear power plant under construction in the USA it is years behind schedule and hundreds of millions of dollars over budget and all involved firms are bankrupt). I am old enough to remember the time when silly people who ignored costs supported photovoltaic cells instead of nuclear reactors. I am very old and the relative costs have changed.