Rock. Silicates absorb CO2 through weathering. Olivine absorbs faster in seawater. Weathering is one of the slow non-biologic sinks for CO2. But it is possible

Rock. Silicates absorb CO2 through weathering. Olivine absorbs faster in seawater. Weathering is one of the slow non-biologic sinks for CO2. But it is possible to make it work faster by crushing olivine and dumping it in shallow ocean.