Geochemical interactions resulting from carbon dioxide disposal on the seafloor

The storage of CO2(liquid) on the seafloor has been proposed as a method of mitigating the accumulation of greenhouse gases in the Earthʹs atmosphere. Storage is possible below 3000 m water depth because the density of CO2(liquid) exceeds that of seawater and, thus, injected CO2(liquid) will remain as a stable, density stratified layer on the seafloor. The geochemical consequences of the storage of CO2(liquid) on the seafloor have been investigated using calculations of chemical equilibrium among complex aqueous solutions, gases, and minerals. At 3000 m water depth and 4°C, the stable phases are CO2(hydrate) and a brine. The hydrate composition is CO2·6.3H2O. The equilibrium composition of the brine is a 1.3 molal sodium-calcium-carbonate solution with pH ranging from 3.5 to 5.0. This acidified brine has a density of 1.04 g cm−3 and will displace normal seawater and react with underlying sediments. Seafloor sediment has an intrinsic capacity to neutralize the acid brine by dissolution of calcite and clay minerals and by incorporation of CO2 into carbonates including magnesite and dawsonite. Large volumes of acidified brine, however, can deplete the sediments buffer capacity, resulting in growth of additional CO2(hydrates) in the sediment. Volcanic sediments have the greatest buffer capacity whereas calcareous and siliceous oozes have the least capacity. The conditions that favor carbonate mineral stability and CO2(hydrates) stability are, in general, mutually exclusive although the two phases may coexist under restricted conditions.