Iron seeding of the oceans to increase producer activity is currently being done but on large enough scales to sequester enough CO2 to make a difference it's basically impossible.
The only one ongoing I know of is Sahara dust being (naturally) blown out to the Atlantic ocean. Otherwise, just talk. Somebody who did an experiment had his results suppressed, crudely, with a false narrative promoted in its place. Why that was seen as necessary, and how it happened, seem even more interesting than the experiment.
