I've mentioned this idea before, as a way of producing fish, but it could also fix huge quantities of carbon onto the ocean floor if done at scale.
You have a specially designed ship stationed at a very non-productive part of the ocean. ( most of the world oceans are ocean desert )
The ship controls a robotic electrical pump, on the ocean floor. The pump stirrs up sediment, and pumps it to the surface through a thin (but large diameter) polythene tube.
When the sediment meets the surface, you get a bloom of algae, which naturally happens whenerver water from the depths upwells. The algae are then the bottom of a food chain, that supports vast clouds of plankton. Besides being fish food, the plankton fix CO2 in their bodies which sinks to the ocean floor when they die, fixing CO2 for thousands of years in a natural way, with no possibility of it getting released in the future. It eventually becomes limestone rock after miliions of years.
The money for the pumping could eventually come from fishing licences, taxing the catch that results from the proliferation of fish, in an area where previously there were none. So it could be self financing, once running.
So a win-win situation, with carbon being stored and food being harvested from what used to be ocean desert.
I realise that this will never happen because of politics and investment problems, but I'm pretty sure it would work. Maybe if the climate really does start giving trouble, it might come into the picture, when the politicians start to panic.