If 2.8 billions tonnes isn't enough, can we just spray additional olivine powder over the surface of the ocean? Presumably it would lock up CO2 in the seawater just fine, allowing more CO2 to be absorbed by the ocean from the air.
Rolling stones; fast weathering of olivine in shallow seas for cost-effective CO2 capture and mitigation of global warming and ocean acidification https://esd.copernicus.org/preprints/2/551/2011/esdd-2-551-2...