Geochemical interactions resulting from carbon dioxide disposal on the seafloor

Abstract

The storage of CO 2(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 CO 2(liquid) exceeds that of seawater and, thus, injected CO 2(liquid) will remain as a stable, density stratified layer on the seafloor. The geochemical consequences of the storage of CO 2(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 CO 2(hydrate) and a brine. The hydrate composition is CO 2·6.3H 2O. 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 CO 2 into carbonates including magnesite and dawsonite. Large volumes of acidified brine, however, can deplete the sediments buffer capacity, resulting in growth of additional CO 2(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 CO 2(hydrates) 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 comunities: 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 CO 2 to CH 4 is slow and redox disequilibrium mixtures of CO 2 and CH 4 are likely. Seismic or volcanic activity may cause conversion of CO 2(liquid) to gas with potentially catastrophic release in a Lake Nyos-like event. The long term stability of the CO 2(hydrate) may be limited: once isolated from the CO 2(liquid) pool, either through burial or through depletion of the CO 2 pool, the hydrate will decopose, releasing CO 2 back into the sediment-water system.