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.