In any case, CO2 injected into the deep ocean is still pretty much sequestered.
Seaweed farms. The seaweed would fall to the ocean floor, where some hope it could sequester lots of carbon.
That wouldn't be buried, so I don't think there was any hope it wouldn't be eaten and oxidized. So the plan there was always to use the biomass as a CO2 pump, not as a means to sequester reduced carbon in sediment.
It depends where it’s dumped. I think a lot of people are looking into dumping it into high salinity anoxic basins based on finding carbon with a long residence time there.