GEOCHEMICAL INTERACTIONS RESULTING FROM CARBON-DIOXIDE DISPOSAL ON THE SEA-FLOOR

The storage of CO2(liquid) on the seafloor has been proposed as a method of mitigating the accumulation of greenhouse gases In the Earth's 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 degrees C, the stable phases are CO2(hydrate) and a brine. The hydrate composition is CO2.6.3H(2)O. 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(hydrate) in the sediment. Volcanic sediments have the greatest buffer capacity whereas calcareous and siliceous oozes have the least buffer capacity. The conditions that favor carbonate mineral stability and CO2(hydrate) stability are, in general, mutually exclusive although the two phases may coexist under restricted conditions. The brine is likely to cause mortality in both plant and animal communities: it is acidic, it does not resemble seawater in composition, and it will have reduced capacity to hold oxygen because of the high solute content. Lack of oxygen will, consequently, produce anoxic conditions, however, the reduction of CO2 to CH4 is slow and redox disequilibrium mixtures of CO2 and CH4 are likely. Seismic or volcanic activity may cause conversion of CO2(liquid) to gas with potentially catastrophic release in a Lake Nyos-like event. The long term stability of the CO2(hydrate) may be limited: once isolated from the CO2(liquid) pool, either through burial or through depletion of the CO2 pool, the hydrate will decompose, releasing CO2 back into the sediment-water system.