It would be an overwhelmingly better idea for such islands to make ammonia, and release that into the sea. That would enable surface algae to bloom over a widely dispersed area, far from land, which would absorb huge amounts of carbon and reduce acidity. They might need to introduce small amounts of iron and other minerals along with the ammonia.
They could also gather carbon -- ideally from up-current water -- for (un-)natural gas, but it is clear which contribution would have the larger effect, on atmospheric CO2, per joule of solar energy.
They could also gather carbon -- ideally from up-current water -- for (un-)natural gas, but it is clear which contribution would have the larger effect, on atmospheric CO2, per joule of solar energy.